I want to second ScottP. The summer that I worked in the park, the NW section of the park was very productive that time of year. My fly box was very limited as I didn't take my vice/materials with me, and minimum wage can only buy so many flies. The good news is that you can get by with just a few flies. Hopper/droppers (Dave's, foam, really anything coupled with a prince or hare's ear nymph), Royal Convertibles, Madam-X, Turk's Tarantula, will be about all you need to get a good mix and get started. It is important to have several sizes as well, sometimes I felt size was as much or more important than pattern. Above all, Yellowstone is the greatest place in the world, and the Lamar, Soda Butte and Slough Creek are simply amazing that time of year.