When I was searching for a smith to thread a 10-gauge for choke tubes a few years ago, the only one I could find equipped to do so was Precision Gunsmithing in King City, north of Toronto. I'm sure that if you have no luck finding one closer to you, he could do the job. Be warned: he is one of the most abrasive, unpleasant, difficult people with whom I have ever had the misfortune to deal. He did beautiful work, reasonably quickly and at a reasonable price, but his "people skills" are sorely lacking.