Hmmm..If it's not on the original soundtrack, you might be unable to get ahold of it. I've run into that several times with score. For example - There's a certain piece of music you want, but the producers chose to leave it off of the album release. It's annoying, but we have to live with it.
If it's not a piece of score, try this: If you have the movie, go to the end credits where the song is listed, and look up the artist/album and try and download it off of MusicMatch or itunes, whichever place you go to. Or buy the artists album off of Amazon.com.