Peanut butter (Erdnussbutter) is not as widely available in Germany as it is in the US. If you cannot do without, check the grocery store for peanut butter made in the Netherlands. A small jar is ca. DM 3.--. Some stores also carry peanut butter from the UK. Avoid the US brands, they are very expensive. You will also find 'organic' peanut butter at most health food stores. It is not cheap, but tastes better.
I heard an American friend complain that German health food stores tend to be a lot more expensive than American health food stores. There are basically two varieties in Germany: Independent stores and "Reformhaeuser" (i.e. chains). Reformhaeuser tend to be a little less expensive and offer a bigger variety of goods, so that might be your best bet.