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.
Sourdough starter would probably be a superior option - lactic acid fermentation is essential for a good kvass
I used sour dough starter on my second attempt and it carbonated way faster than just raisins.
Totally believable