Be careful, some Sony mini-disc players only have a "line in" socket (colour coded with white plastic), and even a powered electret microphone will not provide enough gain for the recorder. In this instance you can get a small audio mixer from Maplin Electronics which will pre-amplify the microphone, but it is a lot of bulk to carry around.
I you do have a microphone input (colour coded red), then buy a powered electret. I use a Sony ECM-MS907, which allows a switchable stereo image from 90 degrees to 120 degrees, but you do need to replace the battery occasionally (usually just after a serious concert!)
Hope this helps
Alan Lafferty
one time broadcast engineer..........