I looked for a good one for ages without any luck - although I did see a mint example for sale on the Bike Museum website once, but was beaten to it by a chap in Australia. Another I found (..on Italian eBay) was in such poor condition it wasn't worth restoring.
My own bike arrived with a single seat fitted (..matched to a long alloy tank), so I didn't even have a base on which to build a dual seat. I finally sourced a longer base (a unit built by Albert Bold), and had a local vehicle upholsterer re-trim it to my specification. Its shape's not an exact match with the original 'Sport' seat, but it suits the bike and in some ways I prefer it.
Were you to find an original seat in good condition, I suspect you'd have to be very quick with the cash!