Check in the town of West Yellowstone. There are at least a couple companies that do half and full day tours of the park. It might be alitle tough to get on with them, a lot of locals who have been doing it for awhile. Plus, iirc, they seem to be kind of on-call, with the more experienced drivers getting first shot, which is fair cause they are there year after year for the companies. But that is where I would start id I was interested in dong that type of work up there.