The small removable cushion was lost overboard and sunk. Where to get replacement cushion? We pull that cushion all the time when entering/exiting and usually just place on the sun pad out of the way. Somehow we must have left it out, and it was lost probably while getting on plane.
It's unlikely that SR will still stock the material. Generally speaking, the SR supports materials for about five years, perhaps a bit longer if that particular boat is still in production. I had a seat cushion replaced just after the five year period, as I recall, and purchased it through my dealer. So, start with your dealer. If they say it's no longer available, try and find out who supplied the vinyl and contact them directly for the material. Your local canvas shop will have to make the cushion. Try asking you local shop if they can match the colors. Also, try looking at flounder pounder (fpmarine.com) and give Google a shot if you haven't already.