Technical 15" tire tubes

I've searched and can't find 15" tire inner tubes with metal stem that's not in the center. I've found 16" but no 15's. It's typically for a motorcycle Any ideas? Thanks

 

I got a pair for my slicks from Summit Racing. Call them. I found them very helpful. Bob

 

A Harley Davidson dealership parts counter.

 

Thanks, ended up using 5.50-600x16's with tr-6 stem. Tires are Goodyear Frontrunners 28".