Many Realtors don't like rent-to-own properties for various reasons--some legitimate, some less so. And some aren't too familiar with the process, so they tend to avoid it.
Many rent-to-own properties aren't listed on the MLS that way. In lots of cases, you (or your Realtor) have to identify possible rent-to-own properties from those that are listed either for sale or for rent. Then you make an offer. That's why a lot of Realtors will tell you--correctly--that there aren't many listed on the MLS. But the flip side of the story is that there are a ton of sellers and a ton of landlords who will accept such an offer if presented to them.
What you want to do is find a Realtor familiar and comfortable with creative financing. Probably someone who's used to working with investors, since many investor deals can involve creative financing. And while there are a number of ways to do that, the simplest sometimes works well: Ask. Just ask.
As for finding rent-to-own properties, here's a blog I wrote on that: http://bit.ly/findaleaseoption You and your Realtor can try some of the techniques described there.
Hope that helps.