I've had some of these "found film" experiences, though never with anything extremely interesting. A couple of old sheets of film turned out to have a nice Pacific Northwest landscape and a picture of the Clark County (Washington) courthouse, circa 1955.
Folders in antique and junk shops seem often to have old rolls in them. The shops usually won't separate the camera from the film, however, and I can't justify too much buying of otherwise uninteresting cameras for the sake of the film. Someone who was into collecting low-end folders could do a lot of this stuff.