Visit Montana's Fish & Wildlife & Parks site. You can click on the region you are planning to visit and then fishing sites and it will tell you most of the streams in the area. Here is the link:
here is a link for the far northwest part of the state.
"Unless one can enjoy himself fishing with the fly, even when his efforts are unrewarded, he loses much real pleasure. More than half the intense enjoyment of fly-fishing is derived from the beautiful surroundings, the satisfaction felt from being in the open air, the new lease of life secured thereby, and the many, many pleasant recollections of all one has seen, heard and done." - Charles F. Orvis