You can try this link at the Library of Congress to see if they have it online, but I think they just started this project, so...:
http://www.loc.gov/rr/geogmap/sanborn/My password and online access at local university doesn't seem to be working anymore (don't think it likes Linux), so I can't look up anything for you. It's a crap shoot if you can find them online, freely available. The NY Public Library has them online for NY, for certain time periods, as well as a few other sources from other cities, libraries, and historical orgs. Your local library or uni one may subscribe to them, or can find out if there is a local source; sometimes access has to be from computers in the library, no remote access.
Sorry that I can't help, unless you need maps of Detroit in the 1890s or 1920s, that I have.