Do yourself a favor and rent (save for a down payment, too, if you need that) until you can qualify for a loan and buy a home in the traditional manner.
Rent with the option to buy has a number of pitfalls for the buyer, and almost always favors the seller.
Contrary to what you might think......you don't simply pay rent, and if you decide to buy the home at the end of the lease period, ALL the rent is applied to the purchase price.
It doesn't work that way.
You will be asked to give an upfront NON-refundable deposit..and also pay an amount of rent over and above the usual rent........only that overage will go towards the purchase, and, if you do not buy the home for ANY reason (ie - you can't get approved for a loan), you will LOSE all that deposit money!
Most sellers who want to, and can, sell now, not later.
That's why there are very few of these kind of listings on the market.
You also have to make sure the seller is solvent, and not in any financial distress, too, as that can happen.
Make sure you understand what it is you're asking for!