Unfortunately, the IRS does not have to give you any tax break (including a basis) that you can't prove with reliable records.
The county will have records you can look at in person, if you want to figure it out.
If the property was always only passed by gift, then your basis today is the basis of the original purchaser, way back when.
If the property was ever inherited, the heirs would have received a stepped up basis equal to the fair market value at the time, but that also can be difficult to prove. And difficult to trace, if it was inherited by (for example) several siblings, who then gave or "sold" their share so one sibling can be the sole owner.
If you have a mortgage, you probably have title insurance, which comes with an abstract of title, that is a copy of every time the property was transferred, possibly all the way back to before your state was officially a state. That abstract may be stored with the title insurance company who prepared the abstract, or with an attorney who helped with the sale, or with the title insurance company. You might try to get a look at that.
Otherwise, you either have to do some hands on research at the county, guess a basis and hope you arent' audited, or go with zero basis.
