I can't seem to find the appropriate package that install's
pip on Nixos?
pip is available as a package on NixOS. For example, for the default Python interpreter, the attribute is
Check the package availability:
$ nix-env -qaP '.*pip.*'
Install the desired version e,g:
$ nix-env -i python3.7-pip-19.0.3
Nix Package Manager Guide : Chapter 19. Main Commands
Essentially install pip via virtualenv
Try using this command
python -m ensurepip --default-pip, after the installation is complete you can update it to the latest version.