Every year, the Pokemon Center releases Fukubukuro (Lucky Bags) which contain an random assortment of official Pokemon Center plushies, T-shirts, mugs, and other products.
Next year will be no different as they are preparing PikaPika Bags (their term for Fukubukuro) to ring in the new year. Besides the usual random assortment of products, each PikaPika Bag will come with a Pikachu, Alola Vulpix, or Rowlet mascot resting in a mini bagpack. There’s no other way to get them other than buying a PikaPika Bag.
The PikaPika Bags will be available exclusively at Pokemon Center locations in Japan from January 1, 2018 for 3704 yen. They tend to run out quickly so better be act fast if you want one.