I have a Sony ECM-DS70P mic which works fine, but it wasn't cheap. You normally need a mic that will take power from the MiniDisc unit (plug-in power). It depends on what sort of quality you're looking for.

The main problem of recording from an audience is that you're picking up everything, comments from people around you, or an excellent recording of them eating crisps! If you can get a line out that's great, or get your mic as close to sound source as you can.