I am will heading this direction soon myself for little Maggie. I read about this on YT...
Friends of Animals | Spay/Neuter Certificates Instructions
It offers a certificate for spaying and neutering. The only thing though, you have to use a vet who participates. They will send you the list in your area.
I need to talk with my vet and find out what the costs are and if they will do anything with this program. I've not looked into any of it yet.