Mary is exactly right..In North Dakota right now the housing market is so tight that when a house comes on the market it can actually be sold within hours of it coming available. Seller's know that, and have no reason to agree to a rent to own. I am not in the Bismarck area, but every local in the central and western part of the state have more people than they have housing for them.
Good luck with your hunt...
Mary is a great Agent, and said she will get back with you if she happens upon something! You can count on her to do that...but do realize it is unlikely she will find any sellers willing to accept your terms.