Not very common but you can find them. Gasoline 404 models predominate with an occasional U series going up for sale. The 404s can be found for reasonable prices. U series are quite expensive by comparison and much harder to find. They sold new ones for a short while ago out of Freightliner dealers but only a few hundred were sold before they were pulled from the market (VERY expensive, even used)
Check ebay from time to time or these two sights.
http://cgi.ebay.com/ebaymotors/Mercedes-Benz-404S-404S-Original-Belgian-1967-Unimog-S-404-Pinzgauer-/230522579049?pt=US_Cars_Trucks&hash=item35ac376869
http://www.offroad-imports.com/
http://www.eurotruck-importers.com/
Overall definitely talk to Kartek at the PA meet next weekend. Guaranteed he knows a lot more then me.
U-Series
"New" Unimog
