Latest release of Yabause can be downloaded from the download page. This page also list binary packages available for various Linux distributions.
You can get older releases of Yabause from the sourceforge website.
In most case, you should use the binary and source packages provided on the download page. There's some case where you may want/need to use the latest code on the repository though.
The main Yabause repository is a GIT repository hosted on Github at url: firstname.lastname@example.org:Yabause/yabause.git You can browse it: http://github.com/yabause/yabause