The steel is typically anywhere from 0.8mm to 1.5mm on cars. As for getting spare steel, try a scrapyard and see if they have any random panels lying around, you might even find one that includes similar shapes to the ones you want to repair. One thing that hasnt been mentioned is prep work, its sooooooooooo much easier to weld thin stuff if the metal is spotless so its well worth cutting out more than you really need to to make the weld quality better and reduce the likelihood of blowthrough.