Depending on what sort of work you want to have done, there are a number of different options.
I’m not sure about rifles or shotguns, but for handguns, you might want to check with the American Pistolsmiths Guild. Ron Power and Jim Clark are charter members.
You might also check with local police departments, and check with local competitive shooters.
Thought it’s been many years since I’ve dealt with any gunsmiths, I’ve been very lucky to have had work done on some handguns by Ron Power (I had a Grandmaster 629, and have a 625-5), Leonard Baity for work done on a 1911, and Cylinder and Slide. I don’t know if Ron is still doing revolvers, but his S&W and Ruger revolvers are absolutely amazing. Leonard Baity built a 1911 “Pin Gun” for me, maybe 20 years ago, incredibly accurate, and his work was superb. Cylinder & Slide is also quite well known, doing very high quality work, but they were especially well known for Colt Pythons and Browning/FN High Power. Of course, for 1911s, there were Wilson Combat, Ed Brown, and Jim Clark.
You could also contact American Handgunner magazine, if they’re still in business. I seem to recall that they used to do regular features on gunsmiths. Or maybe Brownells or PowerCustom, as both sell gunsmithing tools.
Regarding certifications, I knew 2 local guys, both were terrific gunsmiths, both were tool & die makers by trade, but had no “official” training or certifications, though both knew and hung out with Ron Power, and I’m sure they picked up some great hints from him.
Good luck,
Mark
Do your own work on your own weapons. Deal with it. Who do you trust? I trust myself. Mechanical skills are an aquirred habit