They are called optic mounts or scope mounts, the problem is that the need another point of attachment. Either on the trigger guard or frame. I have a B-square mount for a HK that mounts to the rail and trigger guard. My race gun has a Alchin one sided mount that the frame of the gun was drilled for.
The problem is that the mount has to fit the gun and I don't think anyone will make one for the M&P as it is a production type gun.
If you want a red dot sight, look into a Dr optic or JP optics, they make a small red dot that will fit in the rear dovetail, but its not something you switch back and forth easily.
If you want a fun gun that you can easily mount a red dot to, get a Ruger Mark III with a top rail.