You might phone Kenda in Ohio. They keep
a decent stock of odd size tires and tubes, and if they sold the size OEM for bikes destined for the USA, might have the exact tube available. They may sell you directly, or refer you to a source that stocks it. Another good source ofr odd (and normal size) tires and tubes is Yellow Jersey, in Madison], Wisc. They're good in that if they don't have it they're willing to try to find it, instead of just saying tough.