There are a couple of routes with this, finding the exact item at a premium or finding something similar for less.
In this case, to find aluminum tubing of an exact size, the place is mcmaster-carr or ebay. You will need to know what alloy you want, for something requiring real stiffness, something like 7075 or 6061. It sounds from the article that they were using some soft flexible tubing but if it's too soft, it may not provide support for very long.
The other way, finding something similar, I've heard of people using aluminum arrow shafts and kite shafts. These can be bought new at hunting stores, ebay or if you're lucky someone may have some for free, particularly if they're a little bent. Soft aluminum tubing might be available at the hardware store, but I usually only see copper iceline...