Cards and figurines have a chance of being dropped by NPCs upon death. When they are displayed in a museum, the museum increases rank, attracting more visitors and thus more profit. Museum rank increases by a smaller amount from cards than it does from figures, however. They can't be bought from stores but can be sold in the player's shop.
Cards also have the additional property of being able to be put into decks when they are used. Decks, however, are very rare items, and as there is no way to remove cards from decks and no way of getting any use from cards in decks, it is a better idea to simply save cards and put them into a museum.
Card packs can be bought from Child Wel in Larna for Music Tickets. Also, you can fill your deck with unique cards and turn them in to Ssil the undead witch in the Abode of witch, who will reward you for every 50 different cards. More info can be found in the Live in This World page.
The Omake overhaul mod added a playable demo of the unimplemented card battle minigame in the original version. Pressing F3 will take you straight to the game with a pre-build deck.
The card game utilizes a mana system similar to that of Duel Master/MTG. However, in addition to the the usual creature cards there are also a couple of spell cards with placeholder images (one of which didn't work). Currently it's not known if said spell cards will be obtainable in-game.