Do you mean a topbox? I got one off ebay 2nd hand included the fitting plate etc, got it for £12-£13 if I remember, inc delivery.
If you do buy 2nd hand make sure you get all the fitting plate bits or you may pay as much for that as you do for the box.
Used to be some on google under the shopping tab, last time I looked, when I got mine. Think they were not too expensive at the time.
