The link rkapsi posted is for the core. You'll also need the gui, which is available at:
http://gui.limewire.org/servlets/ProjectSource

Registration on limewire.org has been broken for awhile. We're not too sure what's wrong, and none of us are too keen on playing with the internals of Tigris.. Since registration isn't really required to get the source, it's not that big of a deal. We'll have it fixed someday, hopefully.

I also recommend joining the core & gui developer mailing lists. Instructions for that can be found at:
http://gui.limewire.org/project/www/MailingList.html

limewire.org in general is the website for open-source LimeWire development.

Thanks.