If you can find a place like we have locally, you can do well.
We have a surplus store that handles a lot of surplus steel. Most of it is new or drop off pieces. Their steel area covers about a city block, so they have everything, generally.
They sell by the pound and make one cut free, so you can have them cut a 20 ft stick in two and haul it in a pickup.
When I get ready to build something, I stroll through to see what they have that might work--let it influence my design.
But they always have standard angle and box sizes, such as 1.5, 2, 3, 4, 6 inch sizes on the rack. A lot of it is rusty, but I can deal with that to buy it cheaper.
ron