You can find Sanborn maps online for free by visiting the UCLA Library website. The library has digital Sanborn maps and listings for holdings at other libraries. The Drury University website also has dated Sanborn maps.
Sanborn maps are available through several academic and state institutions, although the particular years through which these maps run vary. The Library of Congress has a searchable database of Sanborn maps in their Sanborn Fire Insurance Maps Online Checklist, which is found in the Geography and Map Division. The collection is an ongoing project and is updated periodically.