A lot of potential rent-to-own homes aren't listed that way. Basically, you have to create them. You either approach a landlord and offer to rent now and buy later, or you approach a seller and offer to buy later and rent now.
Here's a link to a blog I wrote on how to find (and create) rent-to-own and lease-option opportunities: http://www.trulia.com/blog/don_tepper/2010/03/how_you_can_fi
Hope that helps.