Wordorigins.org

Proquest Historical Newspapers is a database service found primarily in libraries. Use on this site is largely restricted to searches of the New York Times, which is available through the San Francisco Public Library.

Dictionary of English Place-Names; Alan Rayburn; Oxford University Press; 1999.

Toponyms of Canada. A useful, if specialized, source.

Harder, Kelsie B., ed. (1976). Illustrated Dictionary of Place Names: United States and Canada. New York, Van Nostrand Reinhold Company.

Dictionary of toponyms of North America. A bit dated, but one of the most comprehensive, single-volume works on the subject available.

American speech (1925- ), Duke University Press.

Quarterly journal of the American Dialect Society.

Newspaperarchive database.

Subscription site with thousands of newspapers dating back to 1759. The search engine is problematic (among other things it does not let you sort the results by date), but the sheer volume of its contents and the reasonable price makes it an attractive resource.