Blender is available as source code on the Foundation site (http://www.blender.org/). From the main page look for the 'Download' section and then for 'Source Code'.
There is just one source, which is then customized depending on the architecture. THe source code is released compressed, either via gzip or via bzip2.
The source is hence available as blender-#.##.tar.gz or as blender-#.##.tar.bz2, being #.## Blender's version.
Blender is developed exploiting the Concurrent Version Server (CVS) system. Sources cab hence be downloaded via CVS access too.
The CVS server also provides daily checkouts, available from the source download page, as well as as the standard cvs checkout mechanism: cvs -z3 -d:pserver:firstname.lastname@example.org:/cvsroot/bf-blender co blender
Once you got the sources, the adventure of building begins.