Numrich has a pistol-grip for it.
http://www.e-gunparts.com/DisplayAd....rSuperSKU=&MC=
It might also be possible to order a stock from CBC.
http://www.cbc.com.br/en_about/
Google also suggests that at least one Magtech/CBC shotgun is a Remington 870 clone. If a buddy of yours has one, you could see if the stocks are interchangable.