You can reverse resolve Nix back down to just the source code links though, which should be enough to build everything if those URLs are available on your local network.

I'm not too familiar with NixOS, but I've been running Gentoo for ages and don't know why you'd need constant internet. Would you mind elaborating?

For installing new things - they assume a working Internet.

Ubuntu and CentOS at least HAD the concept of a "DVD" source, though I doubt it is used much anymore.