Although one can pay exorbitant prices for starter cultures I've never found it necessary as the bacteria are present on the vegetables. Obviously if the vegetables have been washed with a bactericide it won't work, and excessive heat or using brine containing chlorine (ie tap water) will kill the bacteria too. Exposure to air can allow the ingress of undesirable bacteria, moulds, yeast, and their spores. Salt levels are especially important in sauerkraut and similar vegetable ferments.
Sandor Katz explains it best; he's made a career of using natural bacteria for fermentation (and yeast for alcohol and bread). I particularly like the way he explains the difference between biological and culinary uses of the term "fermentation" in this video: