Send them a letter to the address.
If they have moved it will be forwarded to their new address.
Unfortunately a lot of times for lots of different reasons they can't do this.'
I say never never ever never lease to own, rent to own, lease purchase, lease option. There are all kinds of things that can and do go wrong, where you will loose your money. One of the realtors below said get an attorney involved. The very best ones, say don't do it.
If you can't qualify for a loan now, rent cheap for the next 2-3 years. Pay off all debt, save as much money as you can. Eat cheap...then you'll be ready to do it the right way.
Honestly I can tell you I've never seen a lease purchase type deal be successful over the long run, and I've seen lots of them leave both the buyer and the seller in a real difficult situation with both loosing lots of money, although it is usually the buyer who gets shafted.
Let me know if I can help you in any way. I deal a lot with distressed properties.
Virginia Cook Realtors