I got my full length mag tube from a guy on the Benelli forums. This is his post.
FS: U.S. made Benelli M4 Full-Length mag tubes - NEW THREAD - Benelli USA Forums

This guy makes titanium tubes for the M4. it seems to also have a decent following.
US Made Titanium Benelli M4 Magazine Tubes! Available online Jan 1, 2009. - Benelli USA Forums