There are a few accessories for the pokemon mini which could only be purchased from the Pokemon centres in japan sadly now they no longer sell these items.

Here is the official pokemon centre pokemon mini carry pouch, they came in 3 colours (Blue, pink & yellow) and protect you pokemon mini from scratches while carrying it around

Also there was a carry strap for the pokemon mini which attached to the carry pouch or pokemon mini itself which you could store 1 pokemon mini game inside.