I've only camped along the Big Hole river near Wise River, MT. I've driven through the area a few times and noticed there are a bunch of places to camp on public USFS, BLM or State land/ campgrounds/ Fishing Access Sites (FAS), as well as privately owed campgrounds in the area, a lot of which are centered around the Beaverhead and Bighole rivers. You camp at the following FAS north of Dillon along the Beaverhead river - Maidenrock, Salmon Fly, and Brownes Bridge. Check out the USFS and MT Fish Wildlife Parks webpages for more info.
https://www.fs.usda.gov/recarea/bdnf/recreation/camping-cabins/recarea/?recid=11589&actid=29 Check the Central Fishing District.
http://fwp.mt.gov/fish/guide/fasGuide.html
