If you're looking for the seat cowl (rear seat cover) Buellsterparts.com has two, one red and one black from 60-75 bucks, closest color match I've found to Sunfire Yellow has been the Krylon Fusion Sunbeam which was used on my seat rails.
That's a pretty close color but if you really look you can tell the difference. I'm not sure how good the cowl will look being that close to the stock tail color
If your not looking for a seat cowl and looking for the actual seat,check into a Corbin seat. They offer a variety of colors and seating surfaces. They're not cheap but you may be able to get what you want in a high quality seat.
I actually just ordered one but I would like the seat cowl, gives it a smoother look i.m.o. I did just talk to someone from twin motorcycles and they said the can order me one in yellow