I obviously have considered flying but Amtrak was suggested or the greyhound bus with stops on the way. I would love to get more feedback if anybody knows the best thing to do.
Fly. Pay the extra amount. Fly into Bozeman if possible and head to Yellowstone National Park - one of USA's true jewels.
I'd have to recommend flying. It might cost extra, but at least you're not losing days of your time.
I was in Missoula last February and it was magical. Highly recommended! If you're looking to travel throughout Montana, then perhaps the bus is a great thing to consider.
Taking Amtrak will take you several days from the east coast. Montana is a large state, so depending on where you are trying to go you might have to pay a small fortune to fly! If your target is the southeastern mountain area, try flying into Bozeman on Frontier Airlines through Denver. If you are headed further west, try flying into Spokane Washington-you have access to Southwest Airlines there. Otherwise, United has flights to multiple cities, even the small ones. I would just check regularly in various cities-sometimes there is a random cheap flight in the early morning hours or late nights. They usually park a plane at those smaller airports overnight, so they all have a later evening flight that usually isn't full.