Any well-stocked Japanese kitchen needs a bottle of dashi, a salty cooking stock usually made with dried bonito. Dashi is sometimes combined with soy sauce, and the resulting mixture, called dashi-joyu, is commonly used to prepare soups and season a number of ingredients.
As such a ubiquitous part of Japanese cooking, you can buy dashi-joyu at any supermarket. And if you happen to be at a certain few parking lots in Hiroshima or Okayama Prefectures, now you can get it from a vending machine, too.