kefir
Cultured, slightly fizzy drinking milk, thinner than yogurt, with a fresh tang.
Kefir sits in the chilled dairy section of most large supermarkets, and in Turkish and Eastern European shops. If you can't find it, buttermilk or yogurt thinned with a little water will do.
Substitute:
Buttermilk comes closest in tang and consistency. Yogurt thinned with water also works, but it is milder and lacks the slight fizz.