Well to point out the obvious ones, Petsmart as well as Petco have grant
programs. Pretty well any large company in the pet industry may have
grants or may be willing to make donations (provided 501c(3) applies)..
Also if an employee of Wal*Mart volunteers a certain amount of hours
working with said rescue (that's 501--) they can file for a grant once
every 6 months with a payout of $250..

Many large companies have open grants too for non profits. The best bet
would be to start out local to your area where you can visit the
companies (or subsidiarys of) and talk in person with someone in the
position to tell you right off the bat if they have a program and what
it takes to be a part of it.. But local to start is the best way to get
started IMO..

On a side note if you're up to the PR work that it may take, you might
look into if there are any celebrities that live in or were born in your
area/town.. IF you can get through to them or their PR agent you might
just pull off a nice donation.

Good luck on your searches..

Scott--