here in Russia we making Квас (Kvas) mostly in summer, and you can buy in shops (but with many sugar and additives). Try this one 1. (classic)
спойлер
Kvass (from the Slavic verb “kysati” - “to sour”) is a traditional Russian drink based on rye malt or ready-made bread and crackers [1]. The first mention of it dates back to the chronicle of the late 10th - early 11th centuries. It tells how Prince Vladimir, on the occasion of the Baptism of Rus', ordered the people of Kiev to be treated to honey and bread kvass