1) Maybe Craiglist under "For Sale By Owner" They will probably be the only ones willing to wait out a rent to own deal- Real Estate Agents want to get paid at closing.
2) % Is negotiable. Many times the renter pays an extra X%, to be applied to the purchase price at closing in Y months, years, at Z price. If you don't close, you lose the down payment. So often the upfront down payment is NIL
3) If you don't close, you often lose the extra rent money. Basically, that is what the seller gets for keeping it off the market for X amount of time.
4) The problems of doing this are twofold: If the house dramatically rises in value, the seller might not want to keep the deal, since you get all the equity. If the house drops like a stone for X more time, then you paid 20% more for a rental, since you probably won't want it and the bank won't qualify you for the deal.
5) Most rent to own places are closed within a year or two. No wise owner will want to wait any longer.
6) The potential pitfalls from your point of view is an unscrupulous owner who will use the smallest excuse not to close with you in X amount of time. From his/her pov, you might simply balk a paying such an "unfair" price, and try to walk out of paying 20% more for something that is clearly never going to be a good deal. I had a friend who tried to rent to own their place, and the tenants balked at paying the extra. She lost the place, but won in small claims court- a Phyric victory, needless to say.
With prices the way they are now and will continue to be for a couple of years, simply rent someplace cheap, save the extra 20% yourself, and build up a real down payment. Much better for you, probably better for the owner.