You can obtain a current copy of the source code using CVS.
$> cvs -d :pserver:email@example.com:/repository login
$> cvs -z3 -d :pserver:firstname.lastname@example.org:/repository co /modulename/
Note: If you get an error from cvs when using the login command (cvs login: warning: failed to open username.cvspass for reading: No such file or directory) then repeat the login command.
If you already have an existing Sourdough version and want to update:
$> cd <your sourdough directory>
$> cvs -z3 -d :pserver:email@example.com:/repository update -APd
For Windows users, there are a number of graphical CVS clients available such as the very capable and easy-to-use TortoiseCVS. You should also try a current beta version of WinCVS.