The second-and-final volume of the Rex Mundi omnibus editions is officially out and about. It's the way the series was meant to be read.
The best place to get Rex Mundi, of course, is your local comic book shop. If they don't have it, they'll order it for you. You can also get it on amazon, of course, but I'll bet your local shop (unlike amazon!) doesn't hire neo-Nazi security guards to intimidate its employees. If that sort of thing bothers you.