There are a couple, Mile high shooting does everything from custom builds to being the largest dealer/distributor of Accuracy international in the country And awesome people to boot, Gretan Rifles is also based in CO. Also to add to your list of possibles that I would highly recommend is TS Customs out of miller, SD, Perry Custom Gun out of OK, John Beanland, and Josh Kunz of Patriot Valley Arms, the list goes on and on.
The more I'm around it, the more I'm learning chambering a great shooting barrel isn't exactly Voodoo or Rocket science. It's about being patient and methodical in your setup and machining practices. Therefore outside of guys that are known to be bad actors in the business, I'd go for the guy that provides the best customer service and can actually get me a rifle/barrel/or whatever I'm after in the time frame he quotes instead of the usual 2 more weeks like (5 times over, etc).
As for some classes, Check out Colorado Precision Rifle. Brian is one of the nicest guys you'll ever meet, a great teacher and a hell of a shooter. He also teaches for JP enterprises and Thunderbeast
http://cprifle.com/