your inbox is full - couldn't send it so I'm posting it.

Local guy that makes them from (near as a I can tell) 4" aluminum irrigation pipe and fittings, some welding and bearings. Unfortunately, he only sells local to the Brantford/Hamilton area in Southern Ontario, Canada.
I had a really hard time getting one too, then literally stumbled upon this guy 10 minutes from my house.
My main pipe is 31-32 feet long and he said that was as long as he could ever go.