Where can I buy a pistol grip for my Remington 870?

I have a 20 gauge Remington 870 youth modle, but I beilve the fittings are the same as the 12 gauge full size. I just got a new Remington 870 12 gauge, and I want to kind of trick out my old on. Wanting to put a rear pistol grip on it, and possibly a fore grip for the pump. Mainly want the rear one. Also needs to be under 30 dollers. Thanks:)