I'd say very little if anything interchanges with other years, as far as body parts go. Much of the body will interchange with other 1966 full size Pontiacs - front clip, doors, rockers, quarter panels, trunk lid, bumpers, etc. The only real difference between the body of a 1966 GP and a 1966 Catalina is the shape of the rear window and the trim (in particular, the pot metal piece on the rear of the car and the taillights.)
Pontiac Paradise has a 1966 and a 1965 Grand Prix for sale, but the ads have been up for a very long time so I don't know if either car is still available.
http://pontiacparadise.com/shop/for-sale.php