A friend was trying to put on a sling plate on my M&P15OR with the car stock and he lost my Buffer Detent. Does anyone have a extra one that I could buy, Help!!

Call S&W CS they might send you one for free. I have been given small parts a couple of times.

Bushmaster sells a "lost parts kit", and you can either get it directly from them (takes a couple of weeks), or from Miday for $1 more (takes just a few days):

Bushmaster Lost Parts Kit AR-15 - MidwayUSA

I bought one because I managed to lose the detent spring.