Sometimes you can order larger bulky items from the factory and have them shipped inside a new TT that is being delivered to the dealer. We had that done once with 8' long pieces of trim wood. I would guess that a dealer large enough that does more major type repairs would have it in stock.
You might try asking a local specialty plywood if they have or can get some thin plywood with a similar finish. The only problem there might be getting some of the adhesive backed tape the factories use over joints in a color to match.
These guys sell RV ceiling plywood.
all-rite.com The shipping cost would have to be crazy expensive for 4x8' sheets.