For the type of work I do I buy a bit of aluminum flat bar. My last order was a little over 900lbs of 12ft long bars, and I had another 600-700 lbs of full bars on hand plus shorts and cutoff.
The cutoffs I put on shelves, but the 12 footers I really haven't a place for. At the moment I've got the latest order outdoors leaning up against a shipping container, and the rest of the older stock is leaning up against shelves and walls inside the shop. The inside of the shipping container is lined with shelves being used to recover space in the shop, and it's not setup for practical stock storage.
I hate it.
Of course I also buy a little bit of 20' aluminum pipe and tube. That is to long to lean up against anything inside the shop, but one problem at a time.
If you have a storage solution for the standard (maybe only USA standard) 12 foot stock flat bar I'd love to see it. My shop has a 16ft eave height so I can store it vertically anywhere inside that I have the floor space, but I am trying to recover, not lose floor space right now. I am even considering just storing it all outdoors.