Below are links to Google Earth files.
John Collin's kmz Map
from Pilots of America WEBSITE
It shows the actual physical location of ADS-B ground station transmitters.
FAA's kmz file
from this FAA WEBSITE
The FAA's map shows coverage areas. By clicking on boxes, the coverage is shown for various altitudes.
Altitude coverage is a concern for pilots who are in an airport traffic area.
It is too bad that the FAA map does not have a selection for 1000' above the ground because that is the height of many traffic patterns.
Uncheck "RADAR" to see only ADS-B