There’s a fuckload of original midi compositions in a variety of different genres from back in the day at http://xg-central.com/
If you’re playing back the raw midi files you’ll get the best results on old Yamaha hardware like the MU-80 since XG was Yamaha’s general midi extension thingy that they tried to make happen, and all the stuff here was made with that standard in mind. According to the FAQ theres a VST that can play the sounds too but it’s 32-bit only so you might have to jump through some hoops to get either options working on a modern computer.
There‘s some mp3s on there, but not everything has been converted to one.