You get it in a good hardware store, or from a catalog distributor. Farnell has e.g. some stuff under "Engineering Materials". You can also ask a metal workshop in your area, they mostly have material in different shapes on stock. For production quantities, ask a metal bulk supplier.
Sounds like something found in the scrap pile at a machine shop. I doubt you'll find something sized like that for sale, so you'd either have to get it from scrap, or cut a piece from stock aluminum.