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I spent a week in the area last summer, I have a truck camping setup though so my options won't work if you're just in a passenger car and planning on staying in hotels. In Idaho there's some nice areas outside of Sun Valley, heading directly East out of town there's a dirt road with great dispersed camping. Northwest of there there's a big network of very rural areas/dirt roads around Atlanta. Montana you can't go wrong around Glacier and the surrounding wildnerness areas. I dispersed camped right outside the park to the NW near the Canadian border.