The problem is the leasee/tenant (future buyer) usually wants to lock in today's prices and interest rates and does not want to pay for it. In that case it makes no sense for the landlord/owner (future seller) to give up future potential equity to you for nothing in return.
If you are willing to paying enough upfront for the "Purchase Option", then you may be able to find an owner that is willing to sell you the Option. Another approach is to offer to pay a monthly premium on the rent (maybe 20% over market rate) for a fixed length of time. If you cannot purchase by the set date, then you lose the option and whatever you paid for it.
It would make more sense to figure out how to get you qualified to purchase now or in the near future. If there is no way to get you qualified in the near future, then why would you want to pay for an Option that you may never be able to use?