Upon asking, the motorcycle shop I frequented at the time recommended an upholstery guy that had a small shop behind his house. The quality of his work was very good.
The price was more than the $35 previously mentioned. I had a gel pad installed. The gel pad claimed it could be cut and it was. However, where the cut edges of the gel pad came in contact with the vinyl cover, the vinyl slowly over time developed a bubbly texture.
I took the seat back and had the gel padding removed. The seat foam that had been cut out to make room for the gel was replaced with whatever standard upholstery foam the guy used. He did say it was good foam, though. He used this foam in everything from motorcycles seats to chairs. The upholstery foam and new vinyl cover are very comfortable, even for extended days in the saddle. Most comfortable motorcycle seat, to date. Plus, the foam weighs a lot less than the gel pads.
Ask around until you get a line on a good upholstery guy/shop.
Paul
83 R65