FriCAS suurces:

• Links to release tarballs -- https://fricas.sourceforge.net/download.html
• SourceForge download page -- https://sourceforge.net/project/showfiles.php?group_id=200168
• Browse -- https://github.com/fricas/fricas

BuildFriCAS has information on how build FriCAS from source.

For other forks see:

• Axiom Sources
• OpenAxiom Sources