I just looked on the m&m's site and didn't see any invitations.
If you wanted to make something it would be pretty easy though.
Just go to a craft store and purchase pre-cut circles or buy a circle cutter and cut them yourself.
You could then take 2 circles and overlap them slightly and attach them with a little glue. Then to put the M on you could either stamp it, use a sticker or find a die cut letter m.
Then you could write all your info on the back of the m's.
Throw them in an envelope and your all set.
or
You could purchase a tiny party favor box and fill it with m & m's and then put a slip of paper in it with your party details on it. This would obviously require extra shipping, but it would be fun.
My cousin had this dilemma and I ended up making invitations for her because she couldn't find any.
Good luck.
tamie