Your best resource is an interactive GIS map that has City and Census Designated Places (CDP) layers. Government issued maps are always going to be the most accurate, and legal to use as a source. Google, Bing and Mapquest are copyrighted and not valid sources.
In some places, nobody uses those CDP names, and they really shouldn't be on the map, but they're still going to show the error bubbles if you remove them.
Forget Zip Code maps. The USPS has consolidated so many towns and small cities into the nearest large city, they'll be useless.
Most of these that I've seen were caused by people adding city names outside of the city limits, where the No City box should be checked.