Not sure of the exact size but Aloft Hobbies carries metric size ones that I'm sure you could use. I got the 12mm ones to use in place of 1/2" and they work out fine.
Shipping is almost twice as much as a tube. Better order a bunch of them to spread it out. 5 would run you $135 delivered. The 12mm ones from Aloft would only be $73 delivered. Something to consider....
If you a Tap Plastics store (or similar establishment/s) nearby you might find the tube which will work there. Just recently purchased a tube for a Rad Yak 47. Turned out to fit perfectly.
Just a thought,the canvas boat tops use al tubes,good strength to. Theres always a junk boat sittlng around with 20ft of tubes. Most be 5/8 or 3/4 ,the down supports be 1/2 or so.
You might look at flymarketkites.com. When I bought last time they were a considerably better deal than the sites traditionally recommended by the Bros.
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