First off let me say i have been enjoying the game so far. It is a definite change of pace from other MMO's. I just finished up Pyramid of the Sun in Krokatopia and am using the Theurge's deck. 25 slots. I could really use some bigger capacity decks. When do i start finding upgrades?
I'm not sure exactly which quest, but I picked up a deck with 30 slots dropped (I believe) by a boss in Krocotopia. There is also a "hidden" shop in Krocotopia which will sell a 30 slot deck, but it's for Myth School only. To get to the "hidden" shop, check out the transporter behind the shops in the Oasis. It's on a timer, so it probably won't be active. When the hourglasses on the oblisks on weither side of it are lit, it will activate.
Speaking of bigger decks, can anyone say what the largest deck is?
I think that by late Krok or early Marley everyone gets a deck that size (30 slots) via store/drop <again, for drops, check out the wizard101 central link>.
Nomoonaga's Deck is the largest I remember seeing (50 slots, max 3 spells, max 6 death spells, side 13 Death deck, lvl 30)...but the game has 64 slots available, so probably decks go to that size. I have found that above a certain size things get too diluted, so sometimes a smaller deck is better, when forethought is put into the design vs a certain enemy type.