Jeannie Gaudreau @ JMG Stables...Jeannie doesn't have a web page but if you want her info just google JMG Stables, Clinton, MT
Let me know if I can help you w/other details (farrier...vet...friendly face in the area)
ETA: While the showing scene isn't immense, there are 2 or 3 USDF shows every year in Missoula with several more in the surrounding areas. None of my showing-inclined friends seem to have trouble finding places to spend their entry fee fund
Depending on what you're looking for re: barns, there are lots of options, many close to town. Not all of them have an on site dressage trainer, however. I guess it depends on how much you like to drive.
Another one from Missoula here...
I use the Chapeskis; specifically Olivia. She is a very talented rider and trainer. she has started two horses for me- (she actually ended up purchasing one of them from me). She gives an awesome lesson. I am originally from South Florida and I can't say I ever had a better lesson in all my years down there. The barn is a very well run facility as well.