Veterinarians have bulletin boards in their waiting rooms,so put your small ad on paper,perhaps pictures of the kittens on these boards.You must put free because most states do not allow people to sell animals without health certificates.
Oh, that is so hard the mamma will be looking for them. :( If you are giving them all away I guess you should just get it over with. That way she can grieve over them all at one time. I hope that you get her spayed so that this never happens again. If she gets out she will end up pregnant again in a few short weeks. If money is an issue then I can give you some numbers that may help you with spaying her. Good luck.