This one is the serial-to-MIDI interface you're looking for.
http://www.sweetwater.com/store/detail/FastLane/
Incidentally, I have an old MidiMan (now M-AUDIO) MiniMacman Mac serial-to-MIDI interface I bought years ago for use on my parents' Power Macintosh Performa 6220CD. I still use it with my old Quadra 650 from time to time along with Master Tracks Pro.
http://www.sweetwater.com/store/detail/FastLane/
Incidentally, I have an old MidiMan (now M-AUDIO) MiniMacman Mac serial-to-MIDI interface I bought years ago for use on my parents' Power Macintosh Performa 6220CD. I still use it with my old Quadra 650 from time to time along with Master Tracks Pro.