Purveyors and fabricators of metal. Generally speaking, mild steel is only sold in 20' lengths. Stainless steel and aluminum come in 12' lengths. They'll make cuts for you for a nominal fee. They also run a basic fab shop if you need something cut/bent/welded in a particular shape. It's not the cheapest place to buy metal, but it's the most convenient. Cheaper metal can be had outside of San Francisco.