A few ideas
I had all of the seats replaced by a local upholstery shop near Northridge, California. They did a great job, including with high density foam. Any skilled upholstery shop near you should be able to handle the job.
Concerning the aft seats, I called Grady about replacing them, and they did indicate you could get replacement aft seats from a dealer, from the original manufacturer.
That process was taking a bit too long for me, so I decided to improvise. I built a wood base for the aft seats (actually needed it because one of the seat cushions sat on the porta-potty that was installed) with a little more lift, added resin to the base, and then installed Tempress Nav-Style Seats. The base is held down by long strips of industrial strength velcro running along the sides (it works great and the seats do not move unless lifted straight up). The benefit of all of that is that I should be able to replace those seats any time by simply buying new seats (for about $100 each) and screwing them into the base.
The aft seats are very comfortable, they fold (the original aft seat backs were screwed into the boat and exposed to sun full time) and are removable by removing the base, although they sit a little higher than the originals.
