He walks into Walmart or something, buys every pack of cards, takes them back to his house and unpacks each one, taking out the jokers and putting them in a pile, while stacking the full packs of cards in an otherwise unoccupied room. Harley always sighs when Joker starts because he ignores her, but she lets him do it and hangs out with Ivy instead. And I think that one time Batman busts into Joker's place and he's just sitting there on the floor with cards in neat piles everywhere, but Batman wants to fight him, so he jumps to his feet and runs into the room full of boxes of cards and Bruce is like "o shit- bombs???" But under closer inspection, he finds just normal cards - like 5842 packs of them- and he just turns to Joker like, "r u fuckin' serious????" and Joker just sort of shrugs like "wat? u think i just made all those fuckin cards by hand or sumthin??"