If you are looking specifically for a clinic that offers vouchers for individuals, a search tool I really like is fixfinder.org - From my experience working at a spay/neuter clinic, clinics have different grants and vouchers throughout the year, so you might have to call around to see what programs are available. But FixFinder is a good starting point to find free/low-cost spay/neuter services in your area.
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Sarah Fuqua
Director
The Bobo Fund
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