Every single day and night, you can explore Hogwarts' Classrooms. In those classrooms are desks which can be opened and examined for items inside. In some classrooms, a desk will not open. In others, some desks will have nothing inside. But in every classroom, there can be found at least one wizard card and at least 4+ beans. These cards and beans regenerate as different flavors and cards and allow you to consistently get different cards and tons of beans. If you explore every classroom day and night, without acheiving any other cards anywhere in the game, by Night 4, you should easily have all 101 witches and wizard cards, even Albus Dumbledore. I had mine by Day 3, and soon found myself trying to Triples of every card, which I achieved.

Wizard Card Packs consisting on five cards also randomly regenerate themselves every day and night and can be found in Classrooms. They only will regenerate in classrooms but this is more than enough cards by now to complete your collection and obtain the five bars of health.

If you find you are getting the same amounts of cards, trade with ANYONE in the castle until you run out of doubles of your hundreds of cards and will definitely get all 101 cards.