I don't have any experience with Sea Dek but if you simply want to replace your current bolsters with new fabric I would recommend
http://www.bdcanvas.com/ They are local, do nice work and are reasonably priced.
If you're going to keep the boat for a while, rather then replace the vinyl with new vinyl you might look into replacing them with Sunbrella. We've got black Sunbrella bolsters on our 2000 Carolina Classic that are original to the boat. We never do anything to them but hose them off and they look practically brand new, hold up way better then vinyl and don't get the sun fading or black mold like white vinyl does. See picture below.
Not sure if yours are permanently installed but if they are you want to inspect the wood backing and consider replacing with Starboard rather then wood since they will last a lot longer.
Hope that helps and good luck with your project
