I used to hunt that sort of stuff down quite a bit back in the day. Industrial sites, salvage yards, auto wreckers, steel mills, welders / fabrication shops were all sources. That was a long time ago and metal prices are different now.
Proximity to industrial districts and the ability to transport certainly help. For the sake of lightness, a small sheet box with some rusty springs, wires etc had plenty of great results without having to wield around larger stuff.
I recall many fabrication shops in my youth had scrap metal bins near their dumpsters, but they are less common now as metal theft increased. I recall many smaller welding shops etc would just leave remnants near their dumpsters and frequently had 3/8" plate scrap. but that was a long time ago.
Working in trades I knew of many people salvaging, even stealing new metals from employers. It was often assumed many of the underground explosions in metros were scrappers getting into live mains, heh
-- those interested should look up the documentary "Scrapper" for free on youtube about people who salvage active bombing ranges. It's one of the crazier docs I've ever seen.