Try any Farmers' Co-Op store. A lot of the older farm trucks ran the 6.00X16 tube-type bias ply tires, and Co-Op will have them listed in their catalog, even if they don't have them in stock. That's also a popular size for off-road farm equipment, but I'd be a little leery of using an implement-type tire for a highway application. Highway tires need a DOT certification number, and I'm not sure if implement tires have them. Unless it's got "DOT" followed by a number molded into the sidewall somewhere, it's not supposed to be used on public roads.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!