Greg and I have an Akropovic fitted to our GTS 300's, they are loud which is great as it has the desired affect on the traffic. There are other models though so do a bit of googling...:)
Black Widow of the UK, 200 and 230 size in particular. It will be the muffler only as they no longer do the full kit for Vespas. Pure bling.
Our good friend Phil fitted our originally but you could get it put on by Motorcycle Fairings I am sure..as they just had to re-fit mine after a service. :) I would get the black one if I had a choice now..:)