First off, you can not create MIDI files from WAV, MP3, AU, or any other type of file. MIDI files are similar to sheet music. It says when to play a specific note from a specific instrument at a specific time. The other types of audio files say what sound to hear at a given time.
If you want to edit MIDI files, try out AnvilStudio. It's freeware that will let you edit MIDI files.
If you just want to find them, go to Google and search for MIDI. Or, check out the Links Page in the ringer section.