This is what says about replacement seat covers. "
Usually the original seats for your boat are available for around 5 years after the boat models release date, but will vary from manufacturer to manufacturer. These seats, if available, can be ordered from a certified dealership, and are typically on the expensive side.
If you're boat is older than 5 years (or however long the manufacturer makes them available for) or you don't want to fork out an arm & leg for the OEM replacments, the best option to upgrade or restore 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."
I've found several places to get skins made. I have never used any of them so talk to each and make your own decision on who is best for your situation.
Copycat Upholstery
Lost Factory Patterns
American Marine Coverings