I guess you could ask a dealer if you can order actual OEM cushions for your boat, but more likely I think you will probably just have to hunt around various upholsterers, sailmakers, and canvas
workers to see if they can make and cover them for you.
At least in the States that shouldn't be too difficult. We had a hell of a time finding someone to recover our seats in the Caribbean
for two reasons:
1) most of them had very little selection of fabric
covers from which to choose and,
2) They were all booked up at least a month ahead, and we never wanted to stay in one place for that long, just waiting to get covers done. Eventually we had them done in St Martin
(by a sailmaker) while our boat was laid up for a month while I had to go to Canada
for some business meetings.
We used SensaSuede and are very pleased with the result so far: Makeover