You get decor cards either from the store, from the baskets/coffee/event box, from completing tasks (at the top of the screen), and sometimes from buying tips (you either get cards or coffee/lids/pizza/etc). I don’t know if what you get is random or if the tasks & tip bonuses are specific cards. I’m pretty certain that during the ‘Men in the Workplace’ special event that they intentionally didn’t make enough Dwight cards available to automate his desk to try and get people to buy the $10 pack that had enough to automate his desk and get most of the rewards.