You can probably get by with some type of nylon holster just for the class. My advice is to go to a gun store that carries the M&P and a decent selection of cheap holsters. You can probably find a universal nylon holster and they should be willing to let you try to stick their display M&P into it for fit confirmation.
the universal fit holsters are not suitable for IDPA or really much of anything else.
For IDPA you will need a holster that meets their requirements and one that you can holster one handed.
If you happen to want a Kydex M&P holster email me I probably have what you want in stock.
Mike