Ckeck out, www.allyellowstonepark.com, it lists guides from MT & WY.
Allthough Yellowstone is beautiful and the fishing in the park is ok , fish average 12-14 inches and in some areas with catch & release the average is somewhat higher.
For the $15.00 a day permit there are many place's to fish. I would not hire a guide to fish the park unless I wanted to fish for a particular species of fish away from the crowds, mabe a horseback trip for cutts or grayling.Fishing in the park opens around the middle of July , depending on the zone, of which there are 6 zones. Creel and size and limits vary from zone to zone, as do catch and -release, or fly fishing only waters.
Remember , last year over 3 million people took to the park, thats a lot of traffic! You may want to do a guided-float trip outside the park in the many blue ribbon streams of Montana. To list a few, The Madison, The Jefferson,The Big Hole, The Beaverhead, The Missouri, all hold big trout! 4 to 10 lbs are not uncommon.
I hope this was of some help. Let me know if I can be of any more help. I live here and fish the rivers and lakes every chance I get!