I assume you've looked at
Literotica and
Sex Stories Post where they seem to mostly promote non story paysites. I am guessing that it's harder to sell memberships for an all story site.
But there must be a market for it coz there seems to be quite a few
sponsors doing this kind of paysite.
There's quite a few content providers
here but one of your first stops should be
http://www.morgangrayson.com/