The fastest and easiest thing to do is order them from Restoration Specialties.
http://www.restorationspecialties.com/
Download their catalog. Go to page 208. You want the"Rubber Hole Plugs - Depressed Centers - GM Style". I don't recall if it was the 1/2" or 5/8" I got. Think it was 5/8". Measure your holes. They worked perfectly.