I found both dark and light rye bread at the store but it was the factory-made kind. I definitely made mistakes with both batches including adding way too much bread to the light kvass.the bottles need a day to carbonate then we shall see how well they turned out. Not that I have any reference for the flavor.
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
And bread... mostly it doesn't matter, from shop or self made
I had no idea there was rice and oat kvass, thanks for the link.