Foswiki is distributed under the Gnu General Public License, and sources are freely available.
When you download an official Foswiki package from https://foswiki.org/Download, you automatically get most the Foswiki sources in the download package.
Foswiki developers collaborate online using https://foswiki.org, using the Development and Tasks webs, and on github. You can browse and download all the sources at https://github.com/foswiki