For cheap steel tubing look on Craigslist for used weight lifting equipment. Used to be that you could get them for free just by dragging it out of someones basement, now it might cost a couple bucks because everyone is a capitalist.
Either way, there's some nice square tubing in the big multi-function sets that's not terribly thick, but also not paper thin. I got two sets for free and use the weights for hanging off the 3 point right now for ballast. Treadmills are another good source, thicker wall tubing, but a bigger pain to drag out of someones basement because they don't break down as easy and are quite heavy.
Another option I found was someone selling old used pallet racking. The beams were 1/4" thick C-channel, I've picked up a bunch from the same guy for real cheap. Bottom line, the stuff is out there, just have to be patient. But new is crazy expensive right now due to shortages.