Here are two websites that can help you locate changing county and state boundaries. Thank you Pat Dunford for sharing these links with us.
http://www.maps.n2genealogy.com/#county - N2Genealogy.com
http://randymajors.com/p/maps.html - Put in a location, choose a decade, and up will pop (well, it takes awhile to load) the map with the (then) county lines drawn out. To see the name of the county, click on the map.