Many GNU and GNU-adjacent projects will happily list their release tarballs, but make it annoyingly difficult to find the underlying repos that most of them use. Usually the link is squirreled away somewhere in the "contributing" guidelines.
AFAIK https://savannah.gnu.org is the "sourceforge" for GNU projects. I was thrilled when they stood up a GitLab instance but recently locked it down so one can't even browse without being logged in https://emba.gnu.org/explore -> sign in
Surely most open source projects have a link to their source code? Whether it's github, gitlab, sourcehut, or anything else?