there are different programs in different states. I got a 'coupon' when i had my cat fixed years ago. You'll probably have to fill out an application and send it in; it's based on your income. It takes about a month to get it back in the mail. I would suggest that you call your local SPCA; they will know where to direct you.
if you go to google type in your state and NEUTER VOUCHER it will bring up programs in your state. Here's an example of one in Mass