Sanborn Maps are highly detailed maps of towns that were created by the Sanborn Fire Insurance Co. between 1883 and 1951. The idea was to determine the amount of risk there would be in insuring any particular building based on it’s surroundings. (What businesses surrounded it? Were buildings flush against each other or were they set back? etc?) Although the majority of the maps have passed out of copyright, what is easily available is another question. This differs from state to state. Some states or other groups have committed resources to scanning the maps for their area and uploading and maintaining them. ProQuest (a for profit digital content provider) has also scanned the maps and makes them available through a subscription. In Iowa access to the maps is provided through ProQuest. You just have to sign up for a card for the State Library of Iowa. Once you have your card number you can log on for free.
Find the link for the Sanborn Maps and other database sign ins here:
If you live in the state of Iowa, you can get a State Library Card here:
In Missouri, they provide the maps from before 1924 for free through the University of Missouri libraries, without a card number:
Look for Mansfield in 1913.