• David Rumsey Historical Map Collection
• Perry-Castañeda Library Map Collection
• Library of Congress

Are very good resources for all kinds of maps. If you are looking for something very specific, Google Image search is often more useful. Or check websites related to the region. Town websites tend to have good stuff on them sometimes. Or try looking for old books on Google Books or archive.org, they often contain maps as well.

For the Classical period, the Barrington atlas of the Greek and Roman world has wonderful maps.