Source code
The Fabric source code is kept under version control by Git.
The official main-line git repository is located at git://git.savannah.nongnu.org/fab.git and you can browse the source online here: http://git.savannah.nongnu.org/gitweb/?p=fab.git.
There’s also a repository at Github located here: http://github.com/karmazilla/fabric/tree/master
Changes to the master branches are mirrored between these two repositories.
Getting the source
If you have a Github account, then you just press the ‘fork’ button and off you go.
If you want to get the code from the nongnu.org repository, then you can do so by running this command in a terminal:
$ git clone git://git.savannah.nongnu.org/fab.git
Using Git
The Internets are full documentation on how to use Git. One of my favorites is this one: http://www.kernel.org/pub/software/scm/git/docs/everyday.html