You can get lots of beans and Wiggenweild Potions in Fred and Georges bathroom. Play as Ron and go to the back room. Go up to the robes and press X for both sides. Beans and sometimes Wiggenweild Potions will appear. Go through the secret wall, then go back to Fred and Georges. Go to the robes and do the same thing. Keep going back and forth and you will have plenty of beans. Also when using Harry, Hermione, or Ron, fire your attacking spell at a bush. A bean will appear. Continue to do this as many times as desired.