Welcome to CHR. I don't know anyone who regularly posts here that collects parts for sale, but a quick Google "1947 Oldsmobile Rear Fenders For Sale" brought up at least four different listings on eBay, plus some pictures to the side that had links, too. I would think you'd have better luck on one of the Oldsmobile Owner's Club pages. Again, a quick Google "Oldsmobile Owners Clubs" brought up at least seven different links to Olds specific listings. LMC Truck offers Chevy Truck fenders for that era, but nothing for the cars.

Good luck with your search.