I would suggest looking into adult dating sites, personally. I imagine they have a higher turnover rate from free to paid subscriptions, and are therefore more likely to pay for free lead sign ups. There are quite a few posts about the topic already, but also google will return a long, long, long list of adult dating affiliate programs. anywhere from russian brides(pays very well last time I checked) to regular hook-up sites. It's a huge industry these days so it's actually harder to not find anything.