Beans
When you are in free time in the Hogwarts Castle(Not in a lesson or a mission)Go to Guildroy Lockharts room (The defence against the dark arts classroom)and at the far end of the room there should be Guildroys Chair. Try to jam yourself on top of that and then jump onto the balcony and on there should be about 25-30 beans.
Hope I Helped
Happy Gaming ;-)
More tricks, tips and cheats for this game are right here - Harry Potter and the Chamber of Secrets cheats