It depends on who is selling you the bird! You can find a million rare 'birds' on Ebay (their eggs, at least) or Eggbid or any number of places.
Are you talking rare domestics or rare ducks in general? There are probably plenty of wild ducks that may be considered wild. As for domestics, there are none that I can think of off the top of my head.
I think they're actually a hybrid although ... truth be told they might breed true now. If memory serves they were created by Metzer in CA. Not sure if they're accepted into the APA SOP.