Due to the large immigrant population, some ethnic grocery stores sell 2 - liter bottles of kvass, but it is not the same as the stuff sold at street stands that I remember (that tasted more like something between beer and hard cider, and the bottled stuff tastes like... sweet,
bready soda).
Of course, took me a couple of attempts before i got it completely perfect but i got there in the end I tend to add some baking
soda + lemon juice or yeast — although it does not really grow much, it does give it a slightly more
bready texture.