To find free spay and neutering clinics visit a local vet clinic or ask a local rescue. There are many organizations who spay or neuter animals for free or a nominal fee.
The ASPCA and PetSmart Charities advertise locations and dates of free and low-cost spay and neuter clinics. The clinics are held a few times a year in several locations around the country. Love That Cat offers a list of programs for free spaying/neutering for cats organized by state. Some of the programs only apply to low-income families, while others only work with feral cats. The local Humane Society may also offer low-cost spay/neuter programs.