A statement from the gift giver that they gave you a gift of the fence, on X date, and it cost X amount might be sufficient. It depends on how nice your hypothetical auditor is.
Having no documentation isn't the end of the world - as long as the lack of documentation is due to some reasonable cause. Obviously, "I paid some guy under the table" isn't a good reason.
But a gift is a decent reason. A statement from the giver might cover you, since you inherit their basis in the property.