I finally found it! There are three songs from the John Hughes classic, Pretty in Pink, that are missing from the film soundtrack. All of them are from the club scene. Two are from The Rave-Ups--"Positively in Love" and "Rave Up (Shut Up)"--both of which are available for mp3 download at Amazon.
The final missing tune is "Rudy" from the ska band Talk Back. This is an extremely difficult song to find. The one that is on YouTube is not from the movie, but is a more recent version performed by the band Headbone (the current incarnation of Talk Back). I would purchase this version, just to support them, but I didn't see a way to do that on their MySpace page. (I heard they still play live around LA and even play "Rudy" once in a while, so check them out.)
After some investigation, I was able to find a version from the 1980s that is very close to the version used in the movie, thanks to the ska blogs Tone and Wave and Marco on the Bass. They collaborated last year on an awesome podcast focused on rare ska bands that included the tune and I recorded it.
Download the song Rudy from Talk Back in an iTunes friendly m4a audio version!
If you're looking for an m4a to mp3 converter, check out this one. It's free to try and has good reviews.