The SiG552 is easily one of the biggest pain in the ass airsoft rifles to ever have to find replacement parts for. They are few and far between. Since you got it at a department store, I'm guessing it's a CyberGun SiG552. Typically most people wouldn't dare pick up a SiG552 unless they are and experienced player and plan to do some heavy work on it to make it a beautiful work of art. I own one, and just fixing it can be a pain in the ass, more less building it quite right.
Honestly, it's easier, and probably cheaper, to just buy a new gun. I smashed my pistol grip as well, and that thing is nearly impossible to find for replacement.
Primary: Custom WE M16A3
Secondary: KWA MP9
Side: Custom WE M1911A1