Goldman the goldfish.
Leave me alone! My last name isn't Sachs!
(This is in reference to the investment firm Goldman Sachs.)
There are a few clues that this card is actually intended to be a post-game item. This card is in the fourth page of cards, which contains exclusively post-game cards except for this one. Furthermore, Mitra never gives a hint about how to obtain this card, although she does give hints about all the other 36 cards in the normal game. Additionally, the Sage's Card Gate in Terminal only requires 36 cards, not 37, so it's possible to complete the game normally without ever getting this card.
On the flip side, this card is relatively easy to get before New Game+, and the Steam achievement called "100%" says you need to beat the game with 37 cards. So indeed this card is part of the normal game, not the post-game.
From Mitra's house in the fields, go one screen up and one left to talk to Miao Xiao Tuan Er the cat. Once the Miao Xiao follows you, go 3 screens right and up to talk to Goldman the goldfish. Once the path is cleared, follow Goldman to his house (see map).
Clean the house by moving all the dust into the whole, and Goldman will grant you access to the card.
If you acquired this card from Finty, cleaning Goldman's house and opening the chest prompts Goldman to say:
Goldman: What? It's not there? That shopkeeper must have stolen it!
Location 1 with GlitchesEdit
If you're using glitches, you can enter Goldman's house before rescuing Icky from the Cardboard Box. In this case, Goldman will not be standing in the way and you can open the chest without any ceremony.