as far as I know, you can git the Triumph gel seat, a Corbin, or Sargeant. That's the only aftermarkets I know of. I'm ok with the stock seat, but I've heard great things about the Sargeant and Corbin!
Personally I love the stock seat. This bike is SOO much more comfortable than my old SV650S it's like night and day. But I do wish the drivers portion of the seat extended a little further back. I've got a big fat ass and I tend to sit on the passenger strap during longer rides. Anybody know what a decent price to pay an upholsterer would be to cut it out further back?