1. Ask your vet first. If they don't offer any programs, call other vets. A good place to start is with the list of S/N vets who participate in the county Spay/Neuter program.
Some vets offer Senior discounts, discounts for low income clients, payment plans, etc.
2. Call local rescue groups who may offer financial assistance.
3. Contact the groups listed HERE. (doc) This list is pretty broad in scope, so find the most relevant ones.
4. Contact local social services agencies. If you can get assistance with other living expenses, you may be more able to pay for vet services.