They are just orings, nothing fancy or special. When I overhauled my 1/2 inch Sportsways manifold i just printed out an oring size chart, overlayed the old oring, found the correct size and then Googled that size to find a seller. If I could remember the size I'd share but can't recall. You can find an oring size chart at http://www.deep-six.com/nitrox/orings.jpg
I do have extra tank valve orings for the 1/2 inch Sportsways of you need any. Mark.
"The diver who collects specimens of underwater life has fun and becomes a keen underwater observer. .. seek slow-moving or attached organisms such as corals, starfish, or shelled creatures." (Golden Guide to Scuba Diving, 1968)