AFAIK there isn't any available anymore. All the cmstorm rapid and other variants as well as the novatouch are all end of life so they won't be produced anymore, so no spare top cases.
Your best bet is finding second hand boards or new old stock in classifieds or auctions sites and salvaging the tops off them for yours.
For the masterkeys range, possibly. But that's something you will have to contact CM directly about. But it's unlikely though.
