I dont know of an exact site with them for sale, but they come up now and again on ebay.
Whats the need for genuine Fiat parts anyway? Ive bought 'Abarth' style spoiler and skirts from www.tuning.co.uk/fiatshop and they seem like theyre going to fit properly (not fitted yet!) Also theyre a whole lot cheaper, money you need when getting them sprayed up
ive seen a spoiler i want on ebay ,but its in germany and i cant be arsed to get it from there ,and it said it was a genuine fiat tail spoiler.ive seen them on a few puntos but never in my car catalogues.