Mikey,
Some of the builder's superstores (Lowes and Home Depot in my area) carry shapes and sheet stock in both cold rolled steel and aluminum; gauge selection is not the best. The price is HIGH...gold would probably be cheaper...but is too soft.
If you need a lot, check with a local sheet metal shop; they will usually sell you what you need.
I've not purchased from the on-line sellers; the initial price is usually better, but the shipping costs eat you up.
Like Tony, I buy most of the stock replacement panels I need from the usual suspects, but there are a few items that are not available, like the "B" posts on the Spridget, or the custom work you might want to do. For example, I fabricated a new recessed pan for a Tilton pedal box that's going into my '62 Sprite just this last week. It's from sheet stock (16 gauge) from Lowes.
Ray