You can also leach tannic acid directly from the leaves of oak trees. Take a wooden barrel, fill with leaves, and acorns and pour boiling water over the leaves. Let the it steep for a week or so and decant. Boil down gently with good ventilation) for higher concentration of acid.

The tip about the eating the acorns are true; you can make some interesting pancakes from acorn flower and they are good to munch on after 3 changes of boiling water and "candied" with melted sugar.