Call a Realtor.
However, not all rent-to-owns are on the MLS. In fact, most aren't. Unfortunately, there isn't a single list anywhere of them. In fact, a lot of them are "created" by talking to people who have houses for sale or houses to rent. You're going to have to do some work.
Here's a link to a blog I wrote on how to find rent-to-own homes: http://bit.ly/findaleaseoption
Hope that helps.