diff --git a/shell.nix b/shell.nix
index 2ae043c..3963630 100644
--- a/shell.nix
+++ b/shell.nix
@@ -1,94 +1,6 @@
-{ command ? "bash --noprofile --norc" }:
 # If you're on NixOS, use me! `nix-shell --pure`.
-with import (fetchTarball {
-  # nixpkgs 23.05 as of 2023/07/19
-  url = "https://github.com/NixOS/nixpkgs/archive/2fadc2426928c844054cd28fabe231ff26a70715.tar.gz";
-  sha256 = "sha256:06hpcqhaaqvd5gjcz2ps9lz6q2sf5fwgl5rwllpyl9x4g5g95ahv";
-}) {};
+{ sources ? import third_party/nix/sources.nix }:
 let
-  wrapper = pkgs.writeScript "wrapper.sh"
-  ''
-    # Fancy colorful PS1 to make people notice easily they're in the Monogon Nix shell.
-    PS1='\[\033]0;\u/monogon:\w\007\]'
-    if type -P dircolors >/dev/null ; then
-      PS1+='\[\033[01;35m\]\u/monogon\[\033[01;36m\] \w \$\[\033[00m\] '
-    fi
-    export PS1
-
-    # Use Nix-provided cert store.
-    export NIX_SSL_CERT_FILE="${cacert}/etc/ssl/certs/ca-bundle.crt"
-    export SSL_CERT_FILE="${cacert}/etc/ssl/certs/ca-bundle.crt"
-
-    # Let some downstream machinery know we're on NixOS. This is used mostly to
-    # work around Bazel/NixOS interactions.
-    export MONOGON_NIXOS=yep
-
-    # Convince rules_go to use /bin/bash and not a NixOS store bash which has
-    # no idea how to resolve other things in the nix store once PATH is
-    # stripped by (host_)action_env.
-    export BAZEL_SH=/bin/bash
-
-    # Allow passing a custom command via env since nix-shell doesn't support
-    # this yet: https://github.com/NixOS/nix/issues/534
-    if [ ! -n "$COMMAND" ]; then
-        COMMAND="bash --noprofile --norc"
-    fi
-    exec $COMMAND
-  '';
+    pkgs = import sources.nixpkgs {};
 in
-(pkgs.buildFHSUserEnv {
-  name = "monogon-nix";
-  targetPkgs = pkgs: with pkgs; [
-    git
-    buildifier
-    (stdenv.mkDerivation {
-      name = "bazel";
-      src = builtins.fetchurl  {
-        url = https://github.com/bazelbuild/bazel/releases/download/5.4.0/bazel-5.4.0-linux-x86_64;
-        sha256 = "1w58m1brwjfwsv48fmd66inry67m4vgb3bwvwmamhdv099v183jg";
-      };
-      unpackPhase = ''
-        true
-      '';
-      buildPhase = ''
-        mkdir -p $out/bin
-        cp $src $out/bin/.bazel-inner
-        chmod +x $out/bin/.bazel-inner
-
-        cat > $out/bin/bazel <<EOF
-        #!/usr/bin/bash
-        export BAZEL_REAL=$out/bin/.bazel-inner
-        function get_workspace_root() {
-          workspace_dir="\''${PWD}"
-          while [[ "\''${workspace_dir}" != / ]]; do
-            if [[ -e "\''${workspace_dir}/WORKSPACE" || -e "\''${workspace_dir}/WORKSPACE.bazel" ]]; then
-              readonly workspace_dir
-              return
-            fi
-            workspace_dir="$(dirname "\''${workspace_dir}")"
-          done
-          readonly workspace_dir=""
-        }
-
-        get_workspace_root
-        readonly wrapper="\''${workspace_dir}/tools/bazel"
-        if [ -f "\''${wrapper}" ]; then
-          exec -a "\$0" "\''${wrapper}" "\$@"
-        fi
-        exec -a "\$0" "\''${BAZEL_REAL}" "\$@"
-        EOF
-        chmod +x $out/bin/bazel
-      '';
-      dontStrip = true;
-    })
-    zlib
-    curl
-    gcc
-    binutils
-    openjdk11
-    patch
-    python3
-  ];
-  runScript = wrapper;
-}).env
-
+(import third_party/nix/env.nix { inherit pkgs; }).env
diff --git a/third_party/nix/env.nix b/third_party/nix/env.nix
new file mode 100644
index 0000000..473e0fd
--- /dev/null
+++ b/third_party/nix/env.nix
@@ -0,0 +1,92 @@
+{ pkgs, extraConf ? "" }: with pkgs;
+let
+  wrapper = pkgs.writeScript "wrapper.sh"
+    ''
+      # Fancy colorful PS1 to make people notice easily they're in the Monogon Nix shell.
+      PS1='\[\033]0;\u/monogon:\w\007\]'
+      if type -P dircolors >/dev/null ; then
+        PS1+='\[\033[01;35m\]\u/monogon\[\033[01;36m\] \w \$\[\033[00m\] '
+      fi
+      export PS1
+
+      # Use Nix-provided cert store.
+      export NIX_SSL_CERT_FILE="${cacert}/etc/ssl/certs/ca-bundle.crt"
+      export SSL_CERT_FILE="${cacert}/etc/ssl/certs/ca-bundle.crt"
+
+      # Let some downstream machinery know we're on NixOS. This is used mostly to
+      # work around Bazel/NixOS interactions.
+      export MONOGON_NIXOS=yep
+
+      # Convince rules_go to use /bin/bash and not a NixOS store bash which has
+      # no idea how to resolve other things in the nix store once PATH is
+      # stripped by (host_)action_env.
+      export BAZEL_SH=/bin/bash
+
+      ${extraConf}
+
+      # Allow passing a custom command via env since nix-shell doesn't support
+      # this yet: https://github.com/NixOS/nix/issues/534
+      if [ ! -n "$COMMAND" ]; then
+          COMMAND="bash --noprofile --norc"
+      fi
+      exec $COMMAND
+    '';
+in
+(pkgs.buildFHSUserEnv {
+  name = "monogon-nix";
+  targetPkgs = pkgs: with pkgs; [
+    git
+    buildifier
+    (stdenv.mkDerivation {
+      name = "bazel";
+      src = builtins.fetchurl {
+        url = https://github.com/bazelbuild/bazel/releases/download/5.4.0/bazel-5.4.0-linux-x86_64;
+        sha256 = "1w58m1brwjfwsv48fmd66inry67m4vgb3bwvwmamhdv099v183jg";
+      };
+      unpackPhase = ''
+        true
+      '';
+      buildPhase = ''
+        mkdir -p $out/bin
+        cp $src $out/bin/.bazel-inner
+        chmod +x $out/bin/.bazel-inner
+
+        cat > $out/bin/bazel <<EOF
+        #!/usr/bin/bash
+        export BAZEL_REAL=$out/bin/.bazel-inner
+        function get_workspace_root() {
+          workspace_dir="\''${PWD}"
+          while [[ "\''${workspace_dir}" != / ]]; do
+            if [[ -e "\''${workspace_dir}/WORKSPACE" || -e "\''${workspace_dir}/WORKSPACE.bazel" ]]; then
+              readonly workspace_dir
+              return
+            fi
+            workspace_dir="$(dirname "\''${workspace_dir}")"
+          done
+          readonly workspace_dir=""
+        }
+
+        get_workspace_root
+        readonly wrapper="\''${workspace_dir}/tools/bazel"
+        if [ -f "\''${wrapper}" ]; then
+          exec -a "\$0" "\''${wrapper}" "\$@"
+        fi
+        exec -a "\$0" "\''${BAZEL_REAL}" "\$@"
+        EOF
+        chmod +x $out/bin/bazel
+      '';
+      dontStrip = true;
+    })
+    zlib
+    curl
+    gcc
+    binutils
+    openjdk11
+    patch
+    python3
+    busybox
+    niv
+    google-cloud-sdk
+  ];
+  runScript = wrapper;
+})
diff --git a/third_party/nix/sources.json b/third_party/nix/sources.json
new file mode 100644
index 0000000..0e9dd95
--- /dev/null
+++ b/third_party/nix/sources.json
@@ -0,0 +1,14 @@
+{
+    "nixpkgs": {
+        "branch": "release-23.05",
+        "description": "Nix Packages collection",
+        "homepage": null,
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "b9fc1843e64c11665ef440d02ecb6b3980a62756",
+        "sha256": "0krzc81cn1vfblhljw9wgx1wkfwlifh9dy3z3c64yhkh3xy6pfji",
+        "type": "tarball",
+        "url": "https://github.com/NixOS/nixpkgs/archive/b9fc1843e64c11665ef440d02ecb6b3980a62756.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    }
+}
diff --git a/third_party/nix/sources.nix b/third_party/nix/sources.nix
new file mode 100644
index 0000000..fe3dadf
--- /dev/null
+++ b/third_party/nix/sources.nix
@@ -0,0 +1,198 @@
+# This file has been generated by Niv.
+
+let
+
+  #
+  # The fetchers. fetch_<type> fetches specs of type <type>.
+  #
+
+  fetch_file = pkgs: name: spec:
+    let
+      name' = sanitizeName name + "-src";
+    in
+    if spec.builtin or true then
+      builtins_fetchurl { inherit (spec) url sha256; name = name'; }
+    else
+      pkgs.fetchurl { inherit (spec) url sha256; name = name'; };
+
+  fetch_tarball = pkgs: name: spec:
+    let
+      name' = sanitizeName name + "-src";
+    in
+    if spec.builtin or true then
+      builtins_fetchTarball { name = name'; inherit (spec) url sha256; }
+    else
+      pkgs.fetchzip { name = name'; inherit (spec) url sha256; };
+
+  fetch_git = name: spec:
+    let
+      ref =
+        spec.ref or (
+          if spec ? branch then "refs/heads/${spec.branch}" else
+          if spec ? tag then "refs/tags/${spec.tag}" else
+          abort "In git source '${name}': Please specify `ref`, `tag` or `branch`!"
+        );
+      submodules = spec.submodules or false;
+      submoduleArg =
+        let
+          nixSupportsSubmodules = builtins.compareVersions builtins.nixVersion "2.4" >= 0;
+          emptyArgWithWarning =
+            if submodules
+            then
+              builtins.trace
+                (
+                  "The niv input \"${name}\" uses submodules "
+                  + "but your nix's (${builtins.nixVersion}) builtins.fetchGit "
+                  + "does not support them"
+                )
+                { }
+            else { };
+        in
+        if nixSupportsSubmodules
+        then { inherit submodules; }
+        else emptyArgWithWarning;
+    in
+    builtins.fetchGit
+      ({ url = spec.repo; inherit (spec) rev; inherit ref; } // submoduleArg);
+
+  fetch_local = spec: spec.path;
+
+  fetch_builtin-tarball = name: throw
+    ''[${name}] The niv type "builtin-tarball" is deprecated. You should instead use `builtin = true`.
+        $ niv modify ${name} -a type=tarball -a builtin=true'';
+
+  fetch_builtin-url = name: throw
+    ''[${name}] The niv type "builtin-url" will soon be deprecated. You should instead use `builtin = true`.
+        $ niv modify ${name} -a type=file -a builtin=true'';
+
+  #
+  # Various helpers
+  #
+
+  # https://github.com/NixOS/nixpkgs/pull/83241/files#diff-c6f540a4f3bfa4b0e8b6bafd4cd54e8bR695
+  sanitizeName = name:
+    (
+      concatMapStrings (s: if builtins.isList s then "-" else s)
+        (
+          builtins.split "[^[:alnum:]+._?=-]+"
+            ((x: builtins.elemAt (builtins.match "\\.*(.*)" x) 0) name)
+        )
+    );
+
+  # The set of packages used when specs are fetched using non-builtins.
+  mkPkgs = sources: system:
+    let
+      sourcesNixpkgs =
+        import (builtins_fetchTarball { inherit (sources.nixpkgs) url sha256; }) { inherit system; };
+      hasNixpkgsPath = builtins.any (x: x.prefix == "nixpkgs") builtins.nixPath;
+      hasThisAsNixpkgsPath = <nixpkgs> == ./.;
+    in
+    if builtins.hasAttr "nixpkgs" sources
+    then sourcesNixpkgs
+    else if hasNixpkgsPath && ! hasThisAsNixpkgsPath then
+      import <nixpkgs> { }
+    else
+      abort
+        ''
+          Please specify either <nixpkgs> (through -I or NIX_PATH=nixpkgs=...) or
+          add a package called "nixpkgs" to your sources.json.
+        '';
+
+  # The actual fetching function.
+  fetch = pkgs: name: spec:
+
+    if ! builtins.hasAttr "type" spec then
+      abort "ERROR: niv spec ${name} does not have a 'type' attribute"
+    else if spec.type == "file" then fetch_file pkgs name spec
+    else if spec.type == "tarball" then fetch_tarball pkgs name spec
+    else if spec.type == "git" then fetch_git name spec
+    else if spec.type == "local" then fetch_local spec
+    else if spec.type == "builtin-tarball" then fetch_builtin-tarball name
+    else if spec.type == "builtin-url" then fetch_builtin-url name
+    else
+      abort "ERROR: niv spec ${name} has unknown type ${builtins.toJSON spec.type}";
+
+  # If the environment variable NIV_OVERRIDE_${name} is set, then use
+  # the path directly as opposed to the fetched source.
+  replace = name: drv:
+    let
+      saneName = stringAsChars (c: if (builtins.match "[a-zA-Z0-9]" c) == null then "_" else c) name;
+      ersatz = builtins.getEnv "NIV_OVERRIDE_${saneName}";
+    in
+    if ersatz == "" then drv else
+      # this turns the string into an actual Nix path (for both absolute and
+      # relative paths)
+    if builtins.substring 0 1 ersatz == "/" then /. + ersatz else /. + builtins.getEnv "PWD" + "/${ersatz}";
+
+  # Ports of functions for older nix versions
+
+  # a Nix version of mapAttrs if the built-in doesn't exist
+  mapAttrs = builtins.mapAttrs or (
+    f: set: with builtins;
+    listToAttrs (map (attr: { name = attr; value = f attr set.${attr}; }) (attrNames set))
+  );
+
+  # https://github.com/NixOS/nixpkgs/blob/0258808f5744ca980b9a1f24fe0b1e6f0fecee9c/lib/lists.nix#L295
+  range = first: last: if first > last then [ ] else builtins.genList (n: first + n) (last - first + 1);
+
+  # https://github.com/NixOS/nixpkgs/blob/0258808f5744ca980b9a1f24fe0b1e6f0fecee9c/lib/strings.nix#L257
+  stringToCharacters = s: map (p: builtins.substring p 1 s) (range 0 (builtins.stringLength s - 1));
+
+  # https://github.com/NixOS/nixpkgs/blob/0258808f5744ca980b9a1f24fe0b1e6f0fecee9c/lib/strings.nix#L269
+  stringAsChars = f: s: concatStrings (map f (stringToCharacters s));
+  concatMapStrings = f: list: concatStrings (map f list);
+  concatStrings = builtins.concatStringsSep "";
+
+  # https://github.com/NixOS/nixpkgs/blob/8a9f58a375c401b96da862d969f66429def1d118/lib/attrsets.nix#L331
+  optionalAttrs = cond: as: if cond then as else { };
+
+  # fetchTarball version that is compatible between all the versions of Nix
+  builtins_fetchTarball = { url, name ? null, sha256 }@attrs:
+    let
+      inherit (builtins) lessThan nixVersion fetchTarball;
+    in
+    if lessThan nixVersion "1.12" then
+      fetchTarball ({ inherit url; } // (optionalAttrs (name != null) { inherit name; }))
+    else
+      fetchTarball attrs;
+
+  # fetchurl version that is compatible between all the versions of Nix
+  builtins_fetchurl = { url, name ? null, sha256 }@attrs:
+    let
+      inherit (builtins) lessThan nixVersion fetchurl;
+    in
+    if lessThan nixVersion "1.12" then
+      fetchurl ({ inherit url; } // (optionalAttrs (name != null) { inherit name; }))
+    else
+      fetchurl attrs;
+
+  # Create the final "sources" from the config
+  mkSources = config:
+    mapAttrs
+      (
+        name: spec:
+          if builtins.hasAttr "outPath" spec
+          then
+            abort
+              "The values in sources.json should not have an 'outPath' attribute"
+          else
+            spec // { outPath = replace name (fetch config.pkgs name spec); }
+      )
+      config.sources;
+
+  # The "config" used by the fetchers
+  mkConfig =
+    { sourcesFile ? if builtins.pathExists ./sources.json then ./sources.json else null
+    , sources ? if sourcesFile == null then { } else builtins.fromJSON (builtins.readFile sourcesFile)
+    , system ? builtins.currentSystem
+    , pkgs ? mkPkgs sources system
+    }: rec {
+      # The sources, i.e. the attribute set of spec name to spec
+      inherit sources;
+
+      # The "pkgs" (evaluated nixpkgs) to use for e.g. non-builtin fetchers
+      inherit pkgs;
+    };
+
+in
+mkSources (mkConfig { }) // { __functor = _: settings: mkSources (mkConfig settings); }