Where can I find pet coupons or free samples?

I am helping out a local rescue group. Currently I only have 3 kittens and they supplied some food/litter but I feel bad taking it when so many other pets need help. I am not rich but I would like to buy my own food for the rescues I keep in my home if I can. If you know of any good places to find coupons or free samples of dog/cat food and litter please let me know. Thanks