It's no coincidence, that Bavaria has the best beer in the world: Since the 16th century there have been strong regulations about brewing beer and how to keep up the high quality of the final product by choosing the ingredients very carefully. One of the oldest laws concerning food and drink are the Bavarian Purity Requirements, decreed by Duke William IV. in 1516: Only water, hops and barley should be used to brew Bavarian beer.
Most companies hand out beer tokens ("Biermarken" in German) to their guests, which can then be exchanged for beer in the respective tents. If you want to buy beer tokens, please contact the tent-owners directly.