Typically, the seats that came with your boat are available for approximately 5 years after the boat model's release date. They can only be ordered from certified dealerships and are usually quite expensive.
If you're boat is older than 5 years or you don't want to fork out an arm & leg for the OEM seats, the best option to upgrade is either to purchase marine-grade vinyl and have your seats professionally re-upholstered, or purchase after-market seats. Typically, re-upholstering seats is cheaper, but can get expensive depending on the condition of the seats' foam. If aspects of the seat other than the vinyl are questionable, we suggest replacing the entire seat.