I got one from my local Triumph shop here in Brisbane Aust. last year. $165.00 for the seat new. The backrest was about the same I think but they only had a couple of the backrest as it was discontinued for some reason. Ithink @Mittzy and @peter k got the last 2.
Apparently I just sold mine to @The Kiwi , even though it wasn't actually for sale. He's a sneaky bugger.
Seats are available new for a great price but sadly no more backrests. Suggest you ring around a few dealers and try your luck.