Search online for a list of stores near you that may sell bathmats. These stores include home improvement stores such as Bed Bath and Beyond. You can also search in large department stores such as Target or WalMart. Look in the store's online catalog under bathroom products to check if they have a "Finding Nemo" bathmat.
Search in smaller local shops that may not have online catalogs. Such shops include pawn shops or family-owned stores.
Search for other people selling the bathmat you want in online trading centers. Check eBay and Craigslist first for the products you want.