sources are available here: http://www.linuxconsulting.ro/xorg-drivers/ and here: http://ncc-1701a.homelinux.net/~linux-sis/index.php?page=Downloads<http://ncc-1701a.homelinux.net/%7Elinux-sis/index.php?page=Downloads>
also it is possible to use precompiled unofficial ubuntu packages with 'sis671' or 'sisimedia' driver with xserver-xorg-core from current debian testing. I think these drivers are based on the same sources
