You can only get them from the join avenue antique shop when buying a box(small box, ordinary box, big box, and super big box) in a C variation shop(you can only get them if you have a C variation shop.
-So just go to join avenue. Then go to the antique shop(if you have one, if not youll have to get one). Then buy a box and hope you get lucky. I suggest saving before you go there because the shop will sell out when you buy something and then you'll have to wait the next day. If you never find one that means you dont have a C variation shop so try replacing the shop with another antique shop and you may eventually find one. It is very difficult and rare to get one in B2, so if possible trade one from another game.
You can also get one by trading from W2(plasma frigate) or B/W(route 13).