However, most aren't listed that way on the MLS. You (and your agent, in most cases) have to identify properties that would be likely lease-option candidates, and then make an offer. Also, there are plenty of properties not on the MLS that would also be good candidates.
In the right circumstances, lease-options can be a win-win for both buyer and seller.
Here's a link to a blog I wrote on how to find lease-options: http://bit.ly/findaleaseoption