Lots of areas, keep driving and you’ll find a nook for yourself
Turn into Headwaters lodge and cabins at Flagg Ranch and take the first right onto Grassy Lake Road. There are 8 designated camping areas along the snake river heading to Grassy Lake Reservoir each camping area has between 1-4 sites a pit toilet picnic tables and fire rings. If you keep driving past camp #8 you enter the National forest and you can pretty much camp at any turnout you like- some spots will have nice fire pits but no other amenities. We ended up out here late on a Thursday night and all the designated sites were taken but we found a great spot to share with some other campers right near Grassy Lake.
We drove back 10 miles but pretty sure it went back another 10-15. Great dispersed camping. Roads were well maintained. Drove the pickup back in for tent camping this August. Would recommend for a close spot by Yellowstone National Park.
Great Yellowstone Basecamp just a few minutes from the entrance.
There are a ton of awesome campsites along snake river with about 2 sites per pull-off. Actually, most of these are a little bit more than a mere pull-off. On gmaps, the sites seem to continue all the way on into Idaho. The one we took, we almost missed because as usual, we arrived at nightfall and we turned onto a dark uncertain path with a lone RV parked off to one side, and sure enough, the was an empty space right there waiting for us, complete with trash, toilet, and even a bear box. Each pull-off has 1 toilet and 1 trash, but has a firepit, table, and bear box for each designated camping spot. The river seemed perfect for fishing. I hope to come back one day.