I searched the web for some Cut Film/Plate Holders (type J) that fit into the right-angle focusing back of my Mamiya Super 23 but could not find any, neither in America nor in Europe. May be the chances will be better to find some in Japan.

Unfortunately I don't know any online dealers for used camera gear in Japan. Can somebody tell me one (or more)?

Years ago I used to buy from Camera.Japan(@bluebottle.com). I mailed to them but the mail could not be delivered.