i second edge of the world. great guys. i was in there last summer about this time, and they hooked me up with a full boating package since i didn't take any gear with me. i think they carried wave sport and jackson when i was in there last year. my guess is that for a six-pack of good libations, you'd get the hookup for free....
MT has been really dry this year, so you might have a hard time finding flow....there's a whitewater page devoted to boating around missoula and more so to brennan's wave, the local play spot in missoula, but i can't seem to find any info on them. you might post on there and find a boating partner.
By the waterside I will lay my head.
Listen to the river sing sweet songs to rock my soul.