I'd expect DPMS does sell carry handles. Assuming you have the shorter height front sight then Bushmaster or ArmaLite should work, too.
Actually if you have the short front sight it makes sense to buy a 0.04" taller post then get a milspec height carry handle or quality flip up. I did this - by buying the taller post Bushmaster sells for under $10 then put it on my ArmaLite. Then bought a $35 Colt pulloff on eBay. It zeroed perfectly and later when I got a Magpul MBUS2 flipup it also worked fine.
Honestly I think it's better to get a flip up rear sight instead unless you're certain you'll never need some of the top rail for an optic, like the RDS.
Lots of us have carry handles that we finally replaced and threw in the parts box. That's why you see used ones for sale. Good CHs are tough so you don't have to worry about horrible accuracy from a used one.