I used to work in a farm implement store through college and we had products by Weasler. They made a variety of different couplers and sleeves. Check also TISCO, Granger and maybe McMaster-Carr. Other than that if you have access to driveline tubing you could make one yourself.
Stephan
Are there any reliable, reasonable computer programs, other than the ones offered by Lochinvar and AOSmith, that will calculate required input in BTU/hr for a variety of conditions, i.e. schools, offices, etc?
Stephan