Outside of a salvage yard, I think you will have to buy it directly from Sea Ray unless you luck out and find one on ebay. You may have difficulty though getting the correct vinyl to match the rest of your seats. Sea Ray still produces the same seat for some of their Cruisers.........my 260DA has the same Captains seat that was in my 2002 240SD.
For the salvage yards try doing a Google search on "Boat Salvage".
Check out these guys...www.fpmarine.com. If you don't see it on their website, feel free to give them a call or shoot them an email. As they have told me in the past - just because it's not on the website doesn't mean they don't have it (or can get it).