Mozart Studies Online - This site by Hideo Noguchi has a good selection of Mozart works. Scroll down past the "Study Issues" to reach the clearly labeled MIDI files.

Mozart Piano Works - These Mozart piano sonatas were all sequenced by Marco Melanson.

Classical Archives - Mozart - Many excellent MIDI files of Mozart's works! After creating a free account (or paying a small fee for more downloads per day), you can click on the MIDI file of your choice to put in your "Downloads" folder. When you're done, access the Download folder and download your MIDI files to your computer.

ON Classical's Mozart MIDI files - An extensive collection of Mozart's works - excellent quality files. To download a file, simply click on the "MIDI" text next to the title of the work. A limit of five files per day may be downloaded for non-members. For 15 Euros (about $19.75 USD), you can have unlimited access to all the files on this enormous site, as well as download them all in a zip file for convenience.