You might also talk to the Humane Society. A few years ago I took a feral female cat to them. They spayed her for free and I returned her to where I caught her.
I'm honestly considering just buying an elasterator and doing it myself
Amazon product
Grab the cat, stick its head in a rubber boot, spread its back legs, balls through the band, and there ya go. He'll walk funny for 5 minutes and 2 days later his balls fall off.