Here is a comprehensive source of all 3,142 counties and equivalent government units in the United States, organized by States, alphabetically, with estimated populations at July 2013. Listed with a column of ID keys consisting of International Committee for Information Technology Standards code numbers.
The data is provided in an Excel spreadsheet accessible by Doxserá DB or transferable to a database file.
tags: cities, counties, states, Excel, user friendly, dropdown boxes, data validation, SQL, ACCESS