I know yu can usually find it in electrical supply houses, as they stock it in addition to the stuff they call Uni-Strut. Some electricians use it for making mounts etc for aattaching lots of runs of conduit to electrical panels etc. Another place is most any industirial supplier that sells cabinets and shelving and pallet materials etc. At one time the military used to use a heap of that stuff. It was all over the place, used for shelving, carts, etc etc. I happened on mine when I was asked to make a heap of materials dissapear on base while they were getting ready for an inspection. 99% of the military are masters at scrounge, as its the only way you really have what you may need when the time comes, unfortunately its frowned on, as your supposed to let the supply system do its job......yea right! Anyhow when inspections rolled around it was common practice to borrow an U S Army duece and a half, from a satellite unit away from a home unit, and load it up with all items that needed to dissapear. Then drive this vehicle and park it somewhere on another miltary base for a day or two until inspectors were gone, and then bring it back. Sometimes folks just decided what was getting pitched out or hid, they did not want those items to return, and to turn it in was too much trouble with the amount of paper work involved, and at that time lots of items had no accountability as they were deemed expendable type, but it was hard to scrap out good newitems as folks would be afraid they would get in trouble for scraping good materials out, so folks would haul it away. No questions asked...just make it dissapear. Not legal my any means but its a very common practice in the military.
We used to have F-4's (Phantoms) and they had huge centerline fuel tanks. When we converted to F-16's the F4 was basically being phased out of military service. LOts of them were put up as display aircraft on pedestals and such, HOwever pedestal display aircraft normally do not have centerline fuel tanks on them. We had a F4 centerline fuel tank laying in the fuel tank farm for years after the last F4 left our base. They wanted it gone. It was purged etc, but unused for years. It eventually went out the gate in the back of a pickup truck, with most of it hanging out the tailend of the truck due to its size. LAst I knew someone was trying to make a sort of one man sub out of it.
Your right I "HATE" dexion! Its like a giant erector set for adults. Too many nuts and bolts, that take time to assemble, where a quick weld bead will attach regular angle iron. Yes you can weld up dexion but it has a galvanize coating, and then there is just something about all those holes I don't particularly like on a finished project. Thats just my opinion however.