Fresh cows milk, skim the cream off, shake in a jar with a tight lid until a butter ball forms. It's not legal to sell raw cows milk in some states, but as far as I know, if you have a friend or neighbor with a cow they can give or sell you milk, as long as it's not from a dairy operation.

Here ya go: http://www.realmilk.com/where1.html

I found a guy who not only sells raw dairy, but all kinds of grassfed meats and wonderful farm raised eggs and beet kvas and other hard to find delights. He was right in my town all along!