How to use different TeX Live mirror?

I am trying to install TeX Live 2019 using the most recent unstable channel. The mirror nixpkgs wants to use at - is blocked on my network. Is there a way to use a different mirror without modifying the file?

look up package overrides and overrideattrs

you could use iptables to rewrite the destination IP.
If there are several texlive mirrors, the url could be rewritten to use the scheme described in pkgs/build-support/fetchurl/mirrors.nix