Many of the online radio stations play songs that were exclusively found on "Disney World Forever" kiosks that used to be located around the Walt Disney World property. They contained music from nearly every ride in every park. Personally, I thought they were great (I mean, where else could you get the Swisskapolka...and if you don't have a copy of the Swisskapolka, you don't know what you're missing

), but they were expensive ($19.00 for a cd of 10 songs). For the cost, you got to choose whatever mix you wanted on your cd. I thought they were great, but supposedly they were too expensive to maintain and Disney got rid of this system several years ago.
Therefore, there really isn't a place to get things like "Frontierland Background Music" or "Old Epcot Entrance Plaza Music." However, like others have said, there are two really great theme park soundtracks out. The newer one is a two-disc set and is really comprehensive if you don't have other soundtracks (if you do, like me, you may find the cd not worth buying). The older one, which can be found on eBay and maybe on
Amazon contains older music like "Universe of Energy," "Tomorrow's Child," from Spaceship Earth, and "The Main Street Electrical Light Parade."