Does anyone know where I can find the latitude and longitude values for county borders in the USA so that I can use google.maps.Polygon to draw counties on a map?
you can get the KML file of the county borders here - http://www.gelib.com/us-counties.htm you can extract the coordinates from this file .... (just select the save link and download from Google Earth instead)
The U.S. Census Bureau has map boundary files that I found are significantly easier to work than GELib files (especially when it comes to extracting counties for a single state). They also offer them at different levels of detail:
https://www.census.gov/geo/maps-data/data/kml/kml_counties.html