Find a local metal scrapping company. You'll be able to find a washing machine or drier, which is made of ~16ga galvanized steel, typically painted white. The 'body panels' are generally removable with a cordless screwdriver, and they're a good source of inexpensive, reasonably good quality sheet metal.
i used 16 ga galvanel sheets from online sheet metal supplier because my bus is Thomas i covered from the outside 2 or3 windows at a time 2 inches over lap front to rear with 2 inches bent apx. 30 degrees under roof opening which i will chalk 22 windows cost me 1490 plus some other pcs. no shipping cost because i order in bulk/sorry got that wrong shipping was 125 flat fee