Do you not have manholes in your streets ?
Who is gonna miss just one or two, I mean the city has thousands of them all over the place........
Seriously.....curbside on trash day is where I find much.
Microwave oven outer casing, washing machine (clothes) have nice big pieces on the outside.
Strip the sheetmetal off, and put the rest out at your curb.
Bedframe angle iron (it is hard though)
I buy from fh brundles...cheap enough for a 3m strip of flat bar and get countless practice pieces of 100mm out of it...3mm is ideal, thick enough to need power but thin enough you need some control behind it for mig / stick or Tig