Purchasing one Rewards (From doing quests) Loot (From Boss Drops)
In your case, you can purchase new ones in Wizard City Bazaar. It's located in Odle Town, just as you enter by the right. Click on the deck icon when you reach inside and talk to the merchant and you should see a long list of decks. To make things simpler, you can click on the "Usable" Box which will show decks you can currently used or that applies to your school.
You can also visit the Shopping District. If you go towards the east, you'll see a Deck Icon on a sign. Go inside and you'll see Elowyn. Interacting with him will show you decks he has available for sale.
Alia Misthaven