AR magazines are a commodity made by a variety of companies, most good, a few not so good - you cant go wrong with pmags - they come in 10, 20, 30, & 40 round mags and you should be able to find them just about anywhere for 12-15 dollars retail. somebody might know a sale for 10 to 12 each. they make a gen 2 30 rounder & now have gen 3 mags in all the capacities but the gen 3 are usually a dollar or 3 more - i usually just get the gen 2 - especially if i notice them on sale -1 dont know if S&W even makes mags for those, i wouldnt be surprised if they shipped pmags
edit - avoid promags - you will probably hear lots of horror stories about those