Did your U.S. ancestors buy land from the Federal Government? You can find out and search by state. This site contains Federal land conveyance records for the Public Land States, including access to images of more than five million Federal land title records issued between 1820 and the present. The site also has images related to survey plats and field notes as far back as 1810. I checked out the site and searched for a distant relative that I know participated in the "land rush" in the Oklahoma territory and found a digital image of the original document showing that the land was transferred to him. Currently their mapping feature is not active, but they do promise to have it functional sometime soon.