Ebay has tons of seats for sale. Depending on the type of seat and the location of the seller, shipping can be kind of high. Also, Overton's, West Marine and Cabela's have lots of seats, as does Bass Pro Shops. With West Marine, you can avoid shipping charges if there is a West Marine near you. If the seat you want is not in stock at the store, West will ship it to the store for free. Another place I have seen on the web that has upholstered seats is
www.veada.com, and they often have free shipping specials.