There are three national forest campgrounds around Seeley Lake [1]: Big Larch (406/677-2233, www.fs.fed.us/r1/lolo [2], mid-May–mid-Nov., $10) is closest to the highway on the east side of the lake; River Point and Seeley Lake (both Memorial Day–Labor Day, $10) are farther around the southern and western sides of the lake, respectively.
Several miles south of Seeley Lake, the state park campgrounds at Placid Lake (406/542-5500, http://fwp.mt.gov [3], May–Nov., $15) and Salmon Lake (May–Sept., $15) are often less crowded than their northern counterparts.
Links:
[1] http://www.moon.com/destinations/montana/missoula-and-northwestern-montana/the-swan-and-blackfoot-valleys/seeley-lake
[2] http://www.fs.fed.us/r1/lolo
[3] http://fwp.mt.gov