PVC Pipe is commonly used for fuselage construction. The desired size is the 2 1/4" and 2 1/2" square variety.
The square 2 1/2" inch downspout pipe (AKA American Gutter Pipe) is used in the Debonair plans and it can be found at Sutherlands Lumber stores in the Texas and Oklahoma area. It has also been found at Do It Best hardware stores. The 10' length of white downspout is made by Genova Building Products and the Genova Model Number is RW200.