A lot of aluminum is used around here. The 0.020 sheets for the skin are common and found in hardware stores. No problems there. Thicker sheet aluminum, up to 1/4 inch thick, is hard to find in small sizes. I go to a metal fabricator and look in their supply and scrap bin. They sell it to me by the pound. I use enough to be on an "industrial user" account and I get a discount. The 1/2 inch by 1/2 inch bar stock seen in the photo is not very expensive and I use a lot of it. I buy standard 12-foot long lengths from the metal fabricator. They are a metal supplier, too.
It is not economical to buy standard lengths of the larger aluminum bars that I used for the triple clamps, tailpiece end, and the front fairing brackets I will make next week. I order the larger bar from Fastenal. They sell it by the foot. This makes much more sense than buying a full bar when I only need a small piece.
It is very important to use good metal. Chinese metal can be of awful quality and it should be avoided. The better suppliers stock metal from reputable sources. I always ask to make sure.
Alloy is important, too. The 2" by 2" aluminum bar stock I ordered today is a good example. It is available in 2024 or 6061 alloys. Volume 2 of the book "The Racing Motorcycle" by John Bradley has a chapter on aluminum and its alloys. The major alloying element in the 2000 series is copper and in the 6000 series it is magnesium and silicon. These additives give the aluminum different properties.
Bradley has a lot of information about these alloys, in summary, the 2024 is heat treatable, has high strength, poor corrosion resistance, does not anodize well, and neither TIG welding, gas welding, or brazing are recommended. The 6061 is heat treatable, has good corrosion resistance, color anodizes well, has moderate strength, and TIG or brazing are recommended. Gas welding is "fair." Strength, weldability, and anodizing are not concerns in this application. Corrosion resistance is. These brackets might get salty and 6061 is the choice. In fact, almost all structural aluminum on this bike is of the 6000 series.