Take a look at
Fowlers Parts here in the UK. The complete bracket is listed as no longer available, but some of the component parts are simply listed as Not in Stock - Back Order. It might be worth checking earlier years too. Fowlers don't always spot that earlier designs may fit on later bikes.
It might be worth a phone call. Failing that, get the part number from the above link and search ebay or breakers yards. I suspect there may be bikes written off that have a fully usable top box carrier.
OR - Quick browse around - I believe that Givi make the Honda top box (I may be wrong). A quick look for their top box carrier revealed
this . I don't know if it is the same fitment, but it might be worth further research / contact Givi. It certainly looks similar.