Just went up through MT, up through Glacier, down through Yellowstone, and all through WY. My brother (who is on here) lives in Laramie.
Stop by Dan Bailey's (fairly historic) fly shop in Livingston, beside Bozeman. If you go up the Yellowstone or Gallatin rivers, among others, away from Yellowstone park you'll have plenty of places, although this time of year they probably have plenty of guides and drift boats out there. Seasoned guide and former shop owner in Cody works at Dan Bailey's and recommended fishing the North Fork Shoshone (out the east gate of Yellowstone) was one of the better places up that way. Everywhere in the park will probably be fished pretty hard.
It was really hot out when we were up there, so if it cools down it should be much better fishing. Outside of that, a lot of mountain streams up out of the way are great for tons of bookies.
Hope that helps some.