You don't mention which M&P15 you have.

The M&P15OR (I have one) gas block has two choices for sling attachment included. The large hole towards the front bottom of the gas block, fits a "push button quick disconnect" type swivel. An old style sling swivel mount can be screwed in the rear most hole in the bottom of the gas block.

If you have a M&P15T (I don't) then you have many rail mount choices. Bravo Co may not always have the best prices, but they have lots of stuff and totally professional service, here's lots of sling mounts and such:
Tactical Slings and Sling Swivels Sling Mounts

For a rear Push Button Quick Disconnect on a std stock, I like the Midwest Industries #10 sling mount:
Midwest Industries MCTAR-10

I really like the Vickers Combat Paddded 2 point sling.
Blue Force gear Vickers Combat Applications Sling
Not cheap, but the quick adjust without any extra sling hanging off is great. They have cheaper models also.

Hope it helps.

Edit - I forgot, there's a sling thread over in the 15-22 forum now. For sling opinions:
Best sling...?