There are two good sources… http://www.redteamwx.com/grlevelx.html and http://www.meteor.iastate.edu/~slincoln/GRS/shapefiles/
Be sure to read any instructions and download any necessary style files. Just note: GREarth does not use Shapefiles.
You can also get a tutorial (including making your own shapefiles here)…
http://imgur.com/a/2Px6P