After a random restaurant stop, a local recommended we go check out big springs campground, which is the headwaters of the snake river and not too far outside of Yellowstone. I am so glad we did! After traveling over random dirt roads from Spencer, Idaho, we arrived to find several open spots for just $15 each. Nice and woodsy, pleasant neighbors, and sites are just big enough. There's bear boxes to use. Just a quick walk away is the headwaters of the snake river, where huge trout swim under a bridge and eat marshmallows and American cheese. Only downside is no showers or flush toilets. Close enough to Yellowstone that we slept in the car so we could wake up at 5 am and grab one of the early morning Yellowstone camp sites. Would definitely go back.