I've unlocked a couple randomly (as far as I can tell), but I'd like to be able to buy them all. Does anyone know a way to unlock them all? And possibly a way to buy multiples of them?
Every 10 wins, a new one will be available for purchase. No, you can't buy duplicates.
Additionally, you can only unlock structure decks that you have won with.
EDIT: Found it! I didn't notice this before since I didn't move on in the game until I won using every single structure deck thinking that there's supposed to be some hidden reward.