I am not sure if you have heard of this program or not:
http://www.spayusa.org/
You enter your address and they will send you a certificate with the closest vet that participates in your area. They promote spaying/neutering of animals at a low cost. My normal vet would have cost me $700 for the neutering of my boys, but instead I will be paying about $250 for them to get neutered. I check out the vet and it is a good place, associated with 2 other animal hospitals/clinics in my area.