Here are a few of the biggest in the U.S.:
http://www.issprops.com/
http://www.weaponspecialists.com/ (and see their Facebook page, here:
http://www.facebook.com/pages/New-Yo...d/189095666629)
http://www.hpr.com/
These two do lots of work on independent and B-movies:
http://moviegunguy.com/
http://propguys.com/
The lobbying group for all of the American armorers:
http://aeaa.us/