I think mine is about 12 gallons? It sits on top of the floor, and the toilet attaches directly to it.
I know that Uwe ordered a custom replacement for his '63, I believe it was from All-Rite. Others have used Inca Plastics for custom tanks. You send them the dimensions and they will fab it to your exact specifications.
Occasionally you'll see one for sale here in the Forums Classifieds or on ebay or craigslist, too. Somebody was selling one about 3 months ago in fact.
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