The source code is distributed with the official release at the same address http://sourceforge.net/project/showfiles.php?group_id=41036, but you can also get it from its most current CVS version; see http://sourceforge.net/cvs/?group_id=41036.
People unfamiliar with CVS or whose CVS access is firewalled can use cvsgrabber (http://cvsgrab.sourceforge.net) to download the current source via HTTP.
To install from the source, run the top-level installation script setup.py:
python setup.py install
As usual, other commands are available, e.g.
python setup.py build
to compile it without installing it into the site-packages directory.