You don't buy decks. You start with the cards for all 4 starters decks (with the one you selected already built). Most other cards you craft in the lab using materials dropped from dungeons (PvE) or fight (PvP). Some cards are dungeon rewards, usually a character reward for clearing a dungeon 10 times. There are also some characters that require using the cafe to transform other characters (which on average costs a lot of materials).