The New York City ASPCA helps city residents get their pets spayed and neutered via a traveling van that goes into neighborhoods so that people don’t have to go find a vet. They have no cost services for feral cats and dogs, and for dog owners that are on public assistance, and low cost for other residents. Their van schedule can be found here. If you know a pet owner that needs to have their cat or dog neutered or spayed, send them to that site to find the schedule for their area.