OOHH . . . . . . Welcome CITIZEN !
In order to even point a direction, you need to collect a piece of info.
Buffer adapters and or tubes have two sides of the fence. its either "mil spec", and all thats available which is plenty.
Or its "commercial", and all thats available which is plenty.
I can't tell ya if the M&P is mil or commercial, Magpul, S&W and someone here might already know.
You have to know what the M&P is before anything. I wouldn't send or buy anything more till you get that.
Best bet . . . call Magpul, they know and sell a bunch every week, and correct mil or commercial issues on what is what.
There is a ton of buffers, tubes, adapters, stocks, etc out there. Good luck pickin one out!