Movie soundtracks... often straight from the film itself, with the added ambience.. Especially on older movies there can be really cool sound effects, like the foley... and often sparse instrumentation that's easy to separate. Another thing movies do which you won't regularly find in normal song is repeating variations of similar motifs. So if you're sampling a nice guitar progression here, might be you can find it again with a different instrument, intensity or even a totally different vibes. Movie composers are often constantly referencing sounds and motifs they establish early on. A melody used in a tense action scene, might be hinted at much later on when the heroes are lounging on the beach in Copacabana. Except now its a super laidback bossa tune.
They also tend to make music that needs to get it's point across instantly, while not over powering the action on screen... another quality that makes it great for sampling.