Any big box hardware store has a handful of steel sheet and L-sections, and a store called "Metal Supermarkets" is pretty common throughout North America which will stock anything a hobbyist welder could dream of, order whatever they don't have, cut to length, etc. There's cheaper, better sources but they are not always accessible to a walk-in shopper with a hand sketch.
For practice, most welding suppliers/metal shops have a bin full of mild steel scraps and cut-offs that they may even let you have for free. (The one I went to even gave me a box for them.)
