It has kind of been suggested already, but asking a small price to put towards spaying the mother can work well in several ways. When I was your age, I took in every miserable stray that showed up. After the first ten or so, my parents were no longer willing to foot the bill on fixing stray cats that might vanish as soon as they were done recovering. If a female had a litter, I would advertise the kittens, adding "please be willing to donate $5 (or more) towards getting their mother spayed." This seemed to be pretty effective in getting other animal lovers to help me out, and weeding out the folks who just wanted a 'disposable' free kitten. Best of luck with the litter.