let
  # for pkgs.applyPatches
  # (if you're using niv this has already been evaluated)
  bootstrapPkgs = import <nixpkgs> { };

  nixpkgsArgs = {
    config = { allowUnfree = false; };
  };
  # https://github.com/NixOS/nixpkgs/pull/81557
  nixpkgsExtraSourcesPatch = bootstrapPkgs.fetchPatch {
    url = https://github.com/NixOS/nixpkgs/commit/7535c42ebf14618e6c32577a7e2150f0cd522803.patch;
    sha256 = "0ph6xzg7j89lcjpxklhvgkvp3qmqhplhnq6p4l84b06h3krcb876";
  };
  importNixpkgs = nixpkgs: let
    nixpkgs' = bootstrapPkgs.applyPatches {
      src = nixpkgs.outPath or nixpkgs;
      patches = [ nixpkgsExtraSourcesPatch ];
    };      
  in import nixpkgs' nixpkgsArgs;
in {
  # replace <nixpkgs> with your preferred Nixpkgs source
  network.pkgs = importNixpkgs <nixpkgs>;
}

let
  # ...
in sources // rec {
  pkgs = importNixpkgs sources.nixpkgs;
  # if you have a nixpkgs-unstable source:
  pkgs-unstable = importNixpkgs sources.nixpkgs-unstable;
}