http://www.google.com/search?q=mature+amateurs
Seems
http://www.sexy-search.com/mature-amateurs/ is a good bet.
http://www.sexy-search.com/addurl.asp
Use Google, Yahoo and MSN search for the first few pages and see which link lists come up the highest for what category of free sites you wish to sell.
That's a great start, anyways.