For that application, I'd trust anybody to do it. Do some googling for a local shop that does "salt bath nitride" or "ferritic nitrocarburizing".
I think the trouble you're going to run into is the batch charge. Most outfits don't want to deal with some schmoe's one little part, so there is a batch charge, which will cover a LOT more than one little pistol barrel. If you find a local outfit, they may do it for free or nearly free.
If you come up empty handed locally, I know of 3 places for sure:
KC Jones (Warren Michigan)
H&M Metal processing (Cleveland Ohio)
MMI Trutec (out west, forget where)
If you're in no particular hurry, I plan to send a batch of odds and ends off to H&M later this year. If you want, you can send the barrel to me (when I'm about to send my batch) and I'll toss it in there with mine.