I have an original circa 1960's Japanese Kamado #3.
I would like to find a metal top vent (cap) that fits.
Does anybody know where I can purchase a top vent?
almost all of the Kamado manufacturers have customer service departments and have parts available online. Measure the outside diameter of the ceramic chimney and start calling (or emailing) around. I know that Visions Grills and BGE both sell after market metal tops, you just need to find the right size. if it is close, but a little big, you can seal it with a collar of stove rope, felt or nomax.
If the ones from visions fit, they are generic stainless or black iron with no logo, but with a small grinder or file and a can of high temp black spray paint, i think that you could probably even "genericifi" one from BGE