Here's a link to a blog I wrote on how to find rent-to-own properties: http://bit.ly/findaleaseoption
The bottom line is that there isn't one single good source. You'll find a few--though not many--listed that way on the MLS. Your agent can find others on the MLS by doing the right type of searches. And many--perhaps most--aren't on the MLS at all. You generally have to look in a lot of different places. And, often, you (or your agent) really has to put one together from a house that's currently listed either for sale or for rent.
Hope that helps.... more