I have been succesful finding good quality midis just by utilizing search engines like google, altavista, etc. It does take time to weave out the good ones from the bad but usually on cetain sites if one is good the rest are usually good as well. If you don't mind having to tweak them a little with sound changes etc. It is really not much work and no investment. Karaoke sites seem to have better composed midis, just look for sites by individuals that are kind enough to share them, actual karaoke sites charge a fee.