Re: Montana... what about the rest of the state?
Montana is a huge state with many different environments. The majority of the trout fishing is done in the areas near the mountains. The NW corner of Montana does have some good fishing opportunities and I would do some google searches for both Tim Linehan and Dave Blackburn. They both provide guide service in that area and it would at least give you ideas of some of the fisheries, which you could then do more research on. I am not plugging their guide services but it will at least give you a place to start for searching out places you could fish on your own.
As far as the Northern portions of Montana you will find a few places but not a lot because it is the praire. We call it the high line in Montana and it is primarily dry land crop producing land with only a few rivers that flow through the area. The Missouri, Milk and Marias are the primary rivers in the northern part of the state and they all generally flow dirty and provide only a few limited options for trout or fly rod anglers. Catfish, walleye and sauger dominate the fisheries in the northern part of the state.
Good luck with your adventure and let me know if you have any other questions about Montana.