AR15 carry handle/rear sights, they are everywhere, and cost depends, varys quite a bit. M&P Sport has a std rail, so all of these should work:

Bravo Co - $109
AR-15 Carry Handle BCM

Weaver - $67
47383 - AR-15 Carry Handle Black Weaver Fits on AR-15 and M4 Upper Receiver Rail

Doublestar
56370 - DoubleStar AR-15 Detachable Carry Handle for Flat Top Uppers MIL-SPEC A2 Rear Sight Black Metal

UTG - $40
ARR-119 - AR-15 Carry Handle w/A2 Rear Sight Black UTG Complete Assembly Fits on Flat Top Upper Receivers

I'm sure there's a few hundred more out there. I've used none of these so can't comment on any.