The plastic one you reference in the picture was not "Official," I believe it was made in Australia in the later 1980's.  The Official Brown cover was short-lived in the Era of the VIC-20 when the first C-64's came out costing $595 USD.  When they dropped to $325 around mid-1983 or so, that cover was dropped.  The poor quality Naugahyde (vinyl over fabric) dried out quickly and cracked becoming useless after about 5 years or so (without constant upkeep or "perfect storage").  You will see the plastic ones (smoky) sold on *Bay with a C-64 or Vic-20 maybe once a year. I've never seen one for an A500.

So, good luck looking!