Cost Basis for Gifted Property

My mom gifted a piece of property to me that has been in our family for over 50 years.  I "believe" the property was gifted to her and my dad from my grandparents.  I sold the property in 2024.

 

How do I to a derive the cost basis for the property?  The search of property records only goes back so far online.  My parents and grandparents are deceased so I can't ask them.