IIRC, the stock deck is 1.5 din (sp?) and most aftermarket headunits are 1 din.
Pretty much anywhere will have adapter kits though. Best buy, Wal-Mart, local audio stores.
Also, don't cut up the stock harness. Get an adaptor to plug into the stock harness and then wire into that. You'll be kicking yourself in the future when you want to pull your aftermarket HU at any point. I've gotten these from Best Buy but I imagine that any reputable audio store would have them too. GM stuff is pretty common