Buy it... you don't need to build it. Check the Q&A forums which point to rule 106 in the manual... section 8.
http://forums.usfirst.org/showthread...ghlight=tubing
As to
where to buy it... try calling around to stores that sell air tools. In Vancouver, BC, I know that Princess Auto and KMS Tools would be happy to help you out, but I am sure you can find something local.
Jason