Summer Sun Garden and Brew in Missoula. It is probably the most most complete homebrew shop in Montana. Huge selection of micro and commercial beers that are hard to find anywhere else in Montana too.
There's a place in Billings that seems to ahve everything we need when we go there, and the staff is very helpful. I'm 250 miles away, so I don't have the details off the top of my head, but hopefully someone can chime in.