But Hal's right, any tire store should be able to order one for you. If it were me, I'd click on the “our dealers” tab at the top of this page and find one local, in my case Keno Tractors. Don't see one local? Call Tommy at Affordable or Ronald at Ranch Hand Supply, they can help you for sure.
When I needed a tube I got one at a local tire shop. I had to go to a couple of tire shops until I found one that didn't specialize in cars. The local tractor shop (Kioti) didn't have them.
I looked into ordering one online, they are widely available. However they are heavy and not that expensive and the shipping was as much as the tube.