Call Big Sky rentals, I know they have an office in West Yellowstone near the museum. We rented a large van from them for our group. Great prices, helpful people. I believe they have other locations. They rent winter equipment, summer equipment, cars, cabins, and the RV auto repair guy is fantastic. Really saved our rear end and our vacation. My highest recommendation. Also, it is much easier to tour Yellowstone and the Tetons in a car. You fit into the turn outs with ease, and you can take the narrow one way roads for some of the attractions.