An absurd amount of land is held by people that don't even know it. They aren't looking to sell generally. Lots of inherited and placed in trusts over generations.

Use q public, or whatever is in your area and search for land recently acquired by an estate. Pretty much that means the owner died, there was no clear line of inheritance do typically a distant relative or random person is selling the estate. Try to buy it.

The other is legal notices and government bid sites. You can buy tax owed land, laws vary by state.