I would think any of the fly shops in town could put you with a good guide. Give a couple of them a call, get some names and call each recommended guide and ask a few questions. Be honest with them about your experience and ask what water they recommend at the time you will be there and what tactics they would expect to work then, i.e. nymphing, Hopper/Dropper, streamers etc., if you have a preference.
I only fished out of West Yellowstone once and it was with a guide named Dave Williams who is now dead, booked thru Bud Lilly's shop which is no longer in business. I wish Dave was still alive as he was one of the best guides I have ever fished with.