You can try "The Armory" on Indian River Rd, or the Lynhaven Indoor range across the street from the Lynhaven mall. Both have gunsmith capabilities of varying extent. I've not used either, so I don't know what kinda quality you'll get.
Whomever you find, ask them how they are going to do it. You can only legally do it by welding it to the barrel, welding a blind pin in place, and real high temperature solder (not the pipe kind). I've heard many at shows, etc. say they just use accraglas, or some other type of resing/epoxy which is totally illegal. Make sure they know what they're doing.
You could just send your upper and barrel off to someone like Kurt's Kustom Firearms here on the board (you'll find him in the Industry section).
To be honest with you, I've never looked for someone to do this kinda work around here. I simply left the muzzle bare on the post-ban.
Ross