It just so happens that I just last week recovered my seat. I learned quite a bit along the way. I had the same problem trying to find a ready made seat cover. So, in Bulleteer fashon, I went to a local fabric store, purchased a yard of "Marine Vinyl" ($19 US). I peeled off the old cover, then cut the threads holding the three panels together. After letting the old vinyl sit in the sun to get hot to flatten out, I then placed the old panels on top of the new vinyl and outlined the pattern with chalk. I then attached the piping to the main panel with staples (you could use pins to hold these in place until sewing). I then used a curved canvas needle and durable thread to sew the piping to the main panel. Once that was complete, I attached the rear panel using the same method (when attaching the piping, start in the center of the panel so it will end up equal on each side). Once I had it sewn, I sprayed some upholstery glue on the foam and vinyl and stretched it into place. The vinyl was held in place with clothes pins until the glue was secure.
One mistake I made was not knowing that the Marine vinyl would stretch in one direction more than in the other, (I should have cut the vinyl so that the stretch direction was from side to side rather than front to back). The attached video shows a similar process to that which I used (the vinyl on the video is the type that stretches in two directions rather than one.
I suppose you could find an upholstery shop to get a proper job done, but where is the fun in that? Attached is photo of my first try.
See the linked video.
https://youtu.be/sANLPfZ9BSY