Seriously, All-Star may have hit on something. If by chance this sign is no longer lighted, or was never lighted in the first place....Home Depot and Lowes, among others, sell a corrugated plastic or fiberglass roofing or siding material in sheets that would work. I've seen it in white, green, and even clear. It also comes in galvanized metal, if that's an option.
I guess it depends on how you're going to use it. If it's for the particular sign you sent the pic of, you're going to have to get the V3 or similar, unless it's no longer lited.