For the cost of those small sheets at your local Hardware Box stores, you would be money ahead to buy a whole sheet of cold rolled steel. I have a contact at a local machine shop that orders 18ga for me in 4 x 8 sheets, I normally pay under $50 per sheet.
The OEM sheet metal is normally 19 ga metal, but it is difficult to find in some areas. With manufacturers working toward the lower end of tolerances to save materials (and money), I think you'll find in many cases the more readily available 18 ga is actually closer to the 19 ga trade size than it is to what should be 18. So if you're looking to fab up some sheet metal parts, buy some 18 ga cold rolled steel, and it should take care of most any sheet metal part you need. For structural components (like floor braces), they are normally made from 18 or 16 guage metal (or thicker), but that will depend on the component's intended purpose.
