Have the kittens get their first set of age appropriate shots. If they're less than 6 months old, they'll need everything else till they're 6 months, then they get rabies. Charge a fee. Base it on what it cost you at the vet. Show them the breakdown of fees. This way they can't complain about the cost, cuz it's what they would've had to come up with out of their own pockets.

Call the town ACO where you live and ask if he has a list of low cost spay/neuter clinics available.

Call the Department of Agriculture and see if there are low cost spay/neuter certificates available to you. I know here in the USA, that's what is available to us.

Good luck.