Look for sites having to do with geography. Government agencies at all levels publish maps. Find them on the Web, gopher, and FTP sites. Weather, travel, and aviation sites also offer maps. Sites that feature information on a country, city, or region frequently include maps.
All images produced by the federal, state, and local governments are in the public domain.
That includes NASA images. The consumer systems are among the best places to find public domain images. Members contribute their own work, often in the spirit of sharing rather than to make money. Clip art is often in the public domain. Do not assume that images are in the public domain if they lack copyright information.