Cheese is one of the very few commercial foods you'll be able to find live (fermentation) bacteria living in, but even then many cheeses won't because the producers save expense by pasteurizing instead of more closely monitoring cheeses to make sure they don't develop molds.

But, there are a few companies who do sell high-quality raw fermented foods, like Real Pickles up here in New England. You'll be able to find healthy bacteria on organic farm-bought produce as well; sauerkraut can be made easily by putting some sliced cabbage in a jar with salt, pounding it down, topping off the jar with water, and capping it for a week.