Here's a legal link:
If you don't mind dealing with MIDI, there is an extensive collection of Mozart - as well as every other composer you can name, and some you probably couldn't - at the
Classical Music Archives. These are all public domain compositions, and the sequencers have given their permission for the files to be hosted.
If you need fast, unlimited downloading, they also offer a subscription service... but non-members are still allowed to get up to 10 files each/every day.
- M.