At the north end of Swan Lake, the Swan Lake Recreation Area (406/837-7500, www.fs.fed.us/r1/flathead [1], mid-May–Sept., $14) includes a Forest Service campground on the east side of the highway and a day-use area with a swimming beach and a boat launch on the west side.
Point Pleasant State Forest Campground (406/754-2301, June–Oct.) is a small, free campground seven miles south of Swan Lake [2].
Links:
[1] http://www.fs.fed.us/r1/flathead
[2] http://www.moon.com/destinations/montana/missoula-and-northwestern-montana/the-swan-and-blackfoot-valleys/swan-lake