Your typical sheet metal (body panels) should be 19 ga cold rolled steel. This is hard to find in some areas, and some steel suppliers won't get it unless you buy in bulk (many sheets). If you can find it locally in small qtys, count yourself lucky. But with the steel manufacturer's trying to cut costs, as everyone else does these days, in many cases you will find the 18 ga CRS (which is more readily available) is rolled to the lower end of the tolerance in sizing, and if so it shouldn't be too much thicker than the 19 gauge and thus a suitable replacement. Floor crossmembers (structural stuff) will likely be 18 or 16.
