You can contact Aircraft Spruce and Specialty in Fullerton, CA... They have an ASTONISHING selection of tubing, sheet steel, aluminum, hardware, etc. originally intended for the aircraft building and repair trade. But ther is no reason why we can't use the stuff too. Also, their tubing is DOM (Drawn Over Mandrel) very smooth on the inside, no internal seam, comes in standard sizes, too.
Good Luck, Bob Mazzullo