Thank GOD! Proper terminology!
Try a local NAPA store, they stock all sizes of the plugs. Or, if you have access, you can tap and plug them with the appropriate-sized socket-headed pipe plug.
It's nice to see someone use the correct terminology, but for postingheader, you may want to try "freeze plugs" as most think that's what they're called.
Good luck.
Tony "Keeper of The Realm" D...