During beta the only deck I got as a drop was quest reward in Marleybone. Otherwise I had to buy the decks at the deck shop for in game gold. The one in the hidden shop on Krokotopia (Mummified Deck) is a nice one but it is not school based.
Most people buy their decks. Later on you MAY get a new deck but the main and best way to get a new deck is to buy one. There is a deck shop in every world.
I've had one Deck drop from an enemy, ever, and it was a Mummified Deck which you can buy from Krokotopia. Not much in the way of statistical data, but they do drop. Just not a lot, it'd seem.
I got a mummified deck in Krocktopia and three of the side quests in Marleybone Knights court gave me 3 decks but I couldn't use one(not my school) so I sold it. As for Moo Shu, I haven't been there yet. It can be luck such as pets or your willingness in the case of Marleybone to do other quests besides the main one.
