First of all, I must praise you for being so responsible and caring about your kittens futures! There are some vets who might neuter them at 12 weeks old, so phone around. It won't hurt to ask about multiple cat discounts, either. If you can't get any of them done before rehoming, draw up some kind of contract sop the new owner has to get the kitten neutered as soon as it is old enough, and check up on them. Good luck!