flat 20 gauge [heavier than used on the body] sheet metal from any sheet metal shop - HVAC places [heating, ventilation, air conditioning] - or check some body shops and see where they get their body metal
"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir "When we tug a single thing on an old truck, we find it falls off" ~ me Some TF series details & TF heater pics