.223 to 9mm conversion...
Contact Rock River Arms or better yet, Bushmaster. They both sell actual 9mm lowers, and *should* have a resource or product to convert existing lowers to 9mm and/or .45.
I haven't tried it, but you may be able to find something in Shotgun News. (magazine rack of Wal-Mart). If its in the FireArms community, it'll be advertised in Shotgun News, or Small Arms Review (published by Dan Shea of Stone Mountain - He's one of the big-wigs of the NFA community).