When I did a fundraiser for a local No-Kill Cat shelter last year, I e-mailed AND followed up with phone calls to all cat food suppliers (like Purina, Iams, Science Diet, etc) and believe it or not I got a HUGE response from most of them!

The biggest response came from Nutro www.nutroproducts.com but maybe because the rep lived locally, but he gave me so many samples, we ended up using the leftovers at the shelter.

Also try your local pet stores (Petsmart & PetCo) they will sometimes give you either a gift certificate or things they no longer sell, etc.