The Forest Service maintains several fee campsites in the area around Trout Lake [1], including Tahklakh Lake ($17) and Ollalie Lake ($14).
The section of the Gifford Pinchot National Forest surrounding Trout Lake, roughly between Mt. Adams [2], Mount St. Helens [3], and Mt. Rainier [4], is dotted with all manner of great spots for dispersed camping. Forest Roads 23, 25, and 90 are good places to begin looking.
Links:
[1] http://www.moon.com/destinations/washington/south-cascades/mount-adams/trout-lake-and-glenwood
[2] http://www.moon.com/destinations/washington/south-cascades/mount-adams
[3] http://www.moon.com/destinations/washington/south-cascades/mount-st-helens-national-volcanic-monument
[4] http://www.moon.com/destinations/washington/south-cascades/mount-rainier-national-park