Congrats on your new M&P 15! Which model did you buy?
There are a bunch of different AR-15 aftermarket grips available. My best advice is to head out to a store and check out a few. Find the one that best fits your hand. Below is a link to a few examples:
AR-15/M4 Pistol Grips | Grips at Brownells
A good AR-15 grip will have a hole to accommodate the safety selector spring. Word of caution; not only is there a spring, there is a little detent (stubby pin) in there too. The spring acts upon the detent. The detent acts upon the safety lever. Do not lose that little detent. It's no fun paying for and waiting for delivery of that little part. I know.
As for me, I prefer the
Magpul MIAD grip. It has replaceable front and back straps which allowed me to get a perfect fit. It has a grip compartment for which you can buy different inserts. I have the emergency 3-round insert in mine.