If you only want to do Yellowstone and parts of Montana (Glacier National Park?) then you would probably do best to fly into Bozeman, MT (about 90 miles from Yellowstone), but I couldn't find any rental wheelchair vans there on a Google search.
Found one in Missoula, MT (265 miles to Yellowstone):
http://www.am-mobility.com/van-rentals.html , and Billing, MT (130 miles from Yellowstone):
http://www.accessiblevans.com/local/...air-van-rental . You would have to look into which of these has the best full-sized jet service, as it is unlikely you could get on a commuter plane. Your next bet would be Salt Lake City (325 miles to Yellowstone):
http://www.accessiblevans.com/locati...air-van-dealer or perhaps Casper, Cheyenne, Jackson Hole, and Rock Springs, WY.
In Glacier NP, the famous and historic Red Bus tours do have some accessible vans; you must book well in advance:
http://www.glaciernationalparklodges.com/red-bus-tours.
Keep in mind that there can be road closures due to weather, and snow can occur any time of year in Glacier, and most of the year in Yellowstone as well. There is information about lodging on both the websites for these national parks, as well as Grand Teton (just south of Yellowstone).
(KLD)