I know if you email a lot of companies (Old Mother Hubbard for one) and ask for samples, they'll either send you some via mail or send you free coupons.
The pet supply store that you buy most of your cat/dog food at, do they ever get returns? Or bags that have split, cans that have dinged? If they don't already work with a rescue, perhaps you can ask them for the stuff they can't sell. The wholesalers refund them their money in exchange for the bar code but they don't take the food back. Hopefully there's a store or two in your area that would be willing to share with you
. And if you don't receive a discount on food already, you should ask for one...you buy such a large quantity, the least the store can do for you is offer a percentage off.
And if that doesn't work, have you thought to buy direct from a wholesaler? There's a place nearby here that I used to purchase dog food, treats, and dog shampoos when I was buying in bulk for a shelter and another rescue. Maybe there's a distributor near you as well who'd be willing to sell to you.