nintendo.com, got the Mario set from there.
Included instructions were in Japanese, but I only read at about a kindergarten level.
Rules included were only for Koi Koi.
I printed off Nintendo's rules from the US site, and watched a few Youtube videos.
https://lovethynerd.com/a-definitive-guide-to-koi-koi/
This site had the best rules description I could find.
Game Grumps played it a few months ago on Switch, and their video actually covered a few fringe cases, like a draw, three cards of a suit in the playing field, and a couple otarial
Hachi-Hachi [八八 or 88] and Hawaiian-style Hanafuda are two other games I've seen that suit more than two players, but I have not tried them.