There is tarballs.nixos.org, which is a content-addressable mirror used automatically by fetchurl. Unlike cache.nixos.org it also works if you use a different store prefix.
Note: the script that adds files to the mirror from the Nixpkgs stable branch was broken for the last few months, but it is running again now. (Tarball mirroring broken · Issue #76 · NixOS/infra · GitHub)