A place that does metal work might have some to sell you, though I don't know if they would. When I worked at Nebraska Boiler in Lincoln in the '70s (70th and Cornhusker) they had dumpsters filled with various shapes and sizes of scrap iron that could be welded together if you are so inclined; angle iron is probably the most common form, but you might find whatever else you're looking for. I guess I don't know what rectangular tube pieces are but what is called box beam might serve your purpose, though it's square, not rectangular. I believe they sold all their scrap iron and I assume they still do, so I suppose if you could find out who they sell it to that would be a good source. If you see a big construction job going on where structural steel is going up chances they have a pile of scrap somewhere, though they might not want you on their property because of liability in case you get hurt or something. But again, whoever has the scrap contract would probably be happy to let you sort through what they have. You might try typing scrap iron into your search engine.
I'm assuming you have access to a welder, torch, metal saw, drill press, and all that, because otherwise wood might be better.
