Purchasing Puyo Puyo eSports
Here's a pastebin from the Puyo Discord on how to make a Japanese Nintendo Account and how to purchase Japanese Yen cards https://pastebin.com/dR9nbeae
Finding the game on the eShop
Access the eShop from your Japanese Nintendo Account, either on your Switch or the web browser.
At the time of this article's writing, Puyo Puyo eSports is the top ranking game on the eShop 😃 so you can go to ランキング (ranking) and buy it there.
If you're reading this article months from now, then try scrolling really far down in the NEW releases tab.
Alternatively, activate the Japanese Keyboard on your Nintendo Switch and search for ぷよぷよ (Puyo Puyo)
How do I buy Puyo Puyo eSports on a PS4?
It's basically the same method. Here's a guide for creating a Japanese Playstation Network account in 2018: vg247