Ken, no repro tanks available at this point - not enough demand. It's tiny; a good few people have fitted one to each side and added a second filler. I'd think a good used tank would be $100 and up plus shipping - you'd need someone who could collect from OzHemi in California.
One thing to look at, would be putting a more modern filler neck into it - they always had a problem with "slosh", so the flapper valve in a new filler neck will greatly help.