Dp2 is right. Rarely are Lease-to-Own deals advertised. Most times you have to create them. And Real Estate Agents aren't really necessary to find one. (In fact....many become more of a hindrence than an asset because they have to look out for THEIR best interest and when THEY get paid.) Bone up on some Website based articles, but.......All you need is a Title Company/Closing Attorney who is familiar with Lease-to-Own contracts.
One of the easiest ways is to call up all the ForRentByOwner's in the local newspaper (or go onto Craigslist). Since they are already trying to rent the house you don't have to overcome the "rent hurdle" as you would with a typical MLS listing or a FSBO. You simply ask them about the property....if it sounds good to you, tell them that you're really interested in it but would rather have the opportunity to buy it at the end of the lease. Look for motivated sellers/landlords if you can. They will be more willing to work WITH you.
Many times you will find that the FRBO was renting it because they couldn't sell it in the first place. The FRBO gets the best of both worlds by having their place rented for 12-36 months by someone who will treat it as if it was their own house(unlike a typical "Renter" who doesn't have a financial interest in the house), and then wants to buy it at the end of the lease! What more could a seller/landlord want?!?!?!?
Lease Options are limited only by your imagination. The late-great Real Estate columinst/journalist Bob Bruss once lease optioned a house......for 10+ YEARS!!!! You can also google his name to do a little more research on the subject of Lease Options.
If you'd like some help finding some, shoot me an email and we will see if we can put something together for you.
Darin is correct: there's plenty of information available both on-/offline about them. You could search for a few of my or his earlier posts here on Trulia, or you could go visit http://www.biggerpockets.com to learn more about them.
Ideally, you probably should work with a knowledgeable pro (an agent, RE lawyer, investor, title company, etc) to help you with your offer.
If you want some assistance, I will be glad to help. Please check my profile and give me a call or email me with what type of house you are looking for.