Shop... no one place is always best.

I get most of my metals from the local metals scrap yards.? I'm in Portland OR and there's one with a good supply of aluminum.? They have pretty much everything but for steel I like one in Salem OR.

I've also had good luck at the new metal houses.? Some have surplus bins other will sell odd ends to the hobbyist at a good discount.? It helps a lot if you take the whole piece... like you need 10" and the one on the shelf is 19.5"... take the whole piece.

Keep an eye out for "good stuff".? If you see a piece of stock that fits the kind of thing you do, grab it... save a trip and a lot of time later.

Greg