You either have to earn decks as a drop from a boss, a reward for a quest or you have to purchase them in one of the Shopping Districts. There is a Deck Shop in Wizard City Sopping District right near the fountain. Be aware that some decks you cannot equip until you are a certain level.
Hope this helps!
Sarai Dragonstone Lvl 49 Ice/Storm (So close, yet so far away!)
There are indeed larger decks as you proceed through the game. Each world has better decks, however there are special decks that are also dropped only by bosses. Please see the world guides from the following link (copy and paste link into your browser):