Tex is right, you can do it yourself. And you're right too... you can buy the seat but then you need all of the rest of the stuff. Here's my experience with it all if it will help...
I pulled all of the old seat off of the frame and wire brushed & painted it all. It was in pretty decent shape. I bought a "Vintage" style seat cover and seat back, hog ring pliers & clips from Brothers ($290 + shipping) And that's without padding... No instructions came with it by the way...
For that kind of cash I was concerned about how nice it would turn out - wrinkles, etc.
I went and saw a boat seat cover guy. He quoted me $350. AND I got to pick out the material & style I wanted... I even had him put in pockets along the front of the seat for sunglasses, etc.
Did it cost me more? Yup. Did I get something a bit different then what you see in the catalogs? Yup. Am I satisfied? Yup.
Click on the link below to see pics of it the finished product.
There's two sides for you!
Good luck!