What I'm building is a different animal. My project is a cute little bolt gun but, there are some choices in the AR area to be had...
Try the following:
http://www.model1sales.com/
http://www.olyarms.com/
http://www.pac-nor.com/
http://www.rockcreekbarrels.com/
http://www.ar15barrels.com/
These aren't in any specific order but, I'm sure any of them would love to help you.
I know that Olympic and Model 1 seem to be listed in the forums and have 300 fireball/ whisper/ 300/221 in their ads but, just ask and you might be surprised who else will do the work for you.