Mac serial MIDI ports were reasonably common on late 80s through 90s synthesisers from Roland, Yamaha and others. Standalone MIDI interfaces also turn up on eBay faily regularly.

You might want to try a Yamaha CBX series - a small, portable keybaord with a wide range of onboard sounds. If you were to go for a separate keyboard controller and a sound module, check out the Roland/Edirol Sound Canvas series for a synth. Oodles of high quality sounds and a few FX.

I should think if you keyword search "Serial" or "Mac" in the appropriate ebay categories, you'll get a good idea of your options.