There is a camp gound south of Broadus almost to the Wyoming boarder right along the Powder river. It is primitive but it is decent. The year we camped there the Montana Game and fish had a guy there assigning spots cause it fills up in a hurry
Some Block Management ranchers will let you camp on their land, some with a camping fee; BLM lands generally allow camping anywhere, but no off road driving; State school lands allow camping within 200 yards of access points, limit camping to 2 nights, open fires only in designated campground fire pits, and no off road driving; Some National Forests allow camping anywhere, others limit camping to designated campgrounds, they often change those rules and vehicle restrictions annually.