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.
Tips wise, make sure you have a large enough container as mine over flowed both times I used that jar. And maybe experiment with additional addins.