General Land Office Records
General Land Office Records were useful before, but with the recent addition of survey plats they're downright irresistible. If your ancestor bought land from the U.S. federal government, it's quite possible you'll find a record of it here. Should you find a land patent you need, jot down the details given under the heading "Legal Land Description," then click over to Survey Search to see what plats are online for that location. In a perfect world the land patents would be directly linked to corresponding surveys, but government websites are forbidden by law from being perfect.