I reinstalled my NixOS two weeks ago, more or less. Now it is 19.03.172392.6d7ed964296 (Koi).
Today I modified some parameters, in order to add gparted to system packages. And now it complains with some sources not being found anymore:
exporting https://svn.unix-ag.uni-kl.de/vpnc (r550) into /nix/store/0x4jdm1dhb593bpgazqclg7np5mb9yp2-vpnc-r550
svn: E170013: Unable to connect to a repository at URL 'https://svn.unix-ag.uni-kl.de/vpnc'
svn: E000110: Error running context: Connection timed out
Now, to make nixos-rebuild use your overlay, the simplest way is to put it in the nixpkgs.overlays config option. There is a literal example in the linked option documentation, but you can also import it from a standalone file like this