Around here any of the home supply places (Lowes, Home Depot) and larger hardware stores have 1/8" tempered masonite. I tend to agree that 1/4" Luan ply will hold staples better, glue better (if applicable), etc, provided you can handle the double thickness on your application. With a closed car tempered masonite should serve well, as you will not have much if any exposure to moisture. For a roadster or convertible I would side with Ken (metal) or Brian (Luan + Thompsons) but would lean more to metal.