For the nixos/nixpkgs source itself, there was some discussion of setting up an official radicle node: Could somebody setup an authorative mirror of nixpkgs on radicle?
For the sources of included packages, I believe I’m right in thinking that the FOD results end up on cache.nixos.org as well?
Generally, though, I think this resilience issue is something lots of people would like to see progress on. @mifka01 produced Distributed Nix Cache Server with Cachix Compatibility recently, and there have been various thoughts about using bittorrent, IPFS etc. The general concensus is that ca-derivations would be the best/easiest way to go about that; there’s also various issues around how you trust what is retrieved that way.