There is always a possibility that lodgings in Yellowstone will become available and is worth checking their site regularly for any cancellations. We recently stayed at the Moose Creek Inn in West Yellowstone which [at that time] offered good value compared to other places in this highly priced town, the rooms were on the small side but were clean and equipped with nice furnishings. It's located in a quiet[er] part of town but is within easy walking distance of town. If you want somewhere clean and comfortable to sleep after a day in the park it's not bad at all, but if you want somewhere with lots of amenities, it won't be the place for you.

Dave.