Quark is not easily available in most non German countries, and it cannot really substituted by ricotta or creamcheese. But then it's not really difficult to make. This is the way we made it at my friend's house in a tiny village in the Black Forest where I spent my summer vacations when I was 13. 1 litre (1 1/8 quart) milk makes about 300 - 350 g (ca. 8 - 12 ounces) quark.