Examples of websites you may visit to post the kittens for sale are http://www.terrific-cats.com/for-sale/ or the Petfinder website. You may also come up with your own website, which you may use to sell them.
The safest place would be to contact an animal rescue group. You could always contact vet offices & use their bullientin board. Be careful who get your kittens. Make sure they go go someone responsible then have the mama spayed.