James Harper (aka Gun Parts Guy) did have some U.S. manufacture L1A1 barrels.  You should check with him (www.gunpartsguy.com) and see if he still has some.  If not, he may be able to put you on to someone who may.  If you strikeout there, then you should go to the FAL Files and post a request there (www.falfiles.com).

 

Good luck in your quest.  Let us know if you were successful.