There are three methods of getting the source for harbour, which method you use will depend on how up-to-date you feel you need to be.
An archive of the entire CVS source is made available on a daily basis from via the SourceForge CVS server. Download http://cvs.sourceforge.net/cvstarballs/Harbour-project-cvsroot.tar.gz. Please be aware that this is an unstable snapshot. Using this source could make all sorts of horrible things happen, it might not even compile. You have been warned.
Also note that the above is a backup of the entire CVS archive, this includes records of all the source changes ever made.
Less frequently a snapshot of the source (not a CVS backup) is made available on the Harbour project web site. If you just want the source and not a backup copy of the entire CVS archive this is what you should download.
Once in a while a stable snapshot of the Harbour source is made available from the download page of the Harbour project web site. These snapshots are referred to as "releases" and are considered stable in that the source will compile and there are no known showstoppers in the compiler, virtual machine or run-time library.
If you like to play with alpha quality software but want to feel safe doing so then a stable release is for you.
The best and most direct method of getting Harbour source is from the CVS server. This is where the Harbour developers work on the source for Harbour (actually, they all work on their own machines, this server gives them, in effect, a virtual shared directory).
If you intend to work with Harbour built from the CVS itself you should really consider subscribing to the developer's mailing list.
Please note that if you do want to help develop Harbour then the last option is the best option (and, in the long term, the only useful option for all concerned).