Is this a DS I or a DS II (both were in production at the time and the seats are different). However, cockpit cushions were a factory option at the time as far as I know but you most likely need to go the custom-made route. I use a pair of "generic" cushions on my 1979 DS II (not the factory optional ones), I forget the brand name (might be "So-Pac"), but I have 2 cushions, each about 3' long and 12" +/- deep, about 1" thick. The 2 sections add up to just a bit shorter than my cockpit seats (cockpit would be almost 8' long, but I added an aft deck like on the DS I and that shortens the useable seat length) We use the same cushions for both sides, that is when we tack.... we move the cushions with us, saves $$ and I found that if alone using just one cushion prevents the leeside one from being blown overboard. Unfortunately, the cushions are no longer sold, the company just makes fenders now. Perhaps "Bottomsiders" would sell a set that will fit the DS?
PS: I'll try to remember to take a picture of mine, and write down the brand.