Disney does sell some soundtracks from time to time. I would check there first, however in my experience their supply is usually lacking.
Here are some of my favs:
1)
Amazon.com: They have a ton of it and some of it is other people selling their old CDs while others are bought right through amazon.
2) Itunes: I found a great deal of Disney music here.
3) Ebay: Often the CD you want is relatively old by Disney standard, you may have some luck there.
4) Disney collectors often sell their excess so sometimes you just have to hunt around
5) The Parks: sometimes you can only buy certain CDs in Disney itself. So if you're there and you see them, scoop them up!!
There is another wonderful way to enjoy Disney muisc (however you can't download it). This is by using Live365- the station is called Mouseworld radio and it is absolutely wonderful. I'm always listening to it. You can get it on your computer or smartphone. It is absolutely free

which makes it even better.