How much are you offering down? We specialize in rent-to-own via lease with an exclusive option to purchase. However, when our clients are putting upwards of 15-20% down, we find that a land contract is better suited for the transaction.
I would imagine your average agent is not going to be experienced in Land Contracts (I could be wrong, this is my experience though). The average seller most certainly does not know this method as a possible viable option when selling. Therefore, properties being marketed as "Owner will do Land Contract" are going to be sparse if that.
With that being said, you'll need to market yourself in such a way to appeal to a seller to hear you out on this type of transaction. Once you have their attention, you'll need to explain it fully but briefly & simply. I would recommend having an attorney they can contact to ask more indepth questions...like the attorney/title company who would be closing the deal.
Hence we are back to what you are offering to put down. I think significant cash up front would catch many seller's attention. Targeting sellers with free & clear properties would be another thing to consider as typically they have much more flexibility.