Kombu is a dried edible kelp, and has a wide range of usage. It can be used to extract dashi broth, cooked with food, or used in a pickling bed to enhance flavor. There are many different kinds and quality.
Kombu has a high concentration of umami inside, so that good quality kombu dashi has a very complex flavor. The quality of broth depends on the quality of kombu and it depends on the origin.
Kombu is mostly cultivated and harvested in Hokkaido, a northern island of Japan. And within Hokkaido, there are several different areas where different types of kombu kelps are grown.
When you use kombu, do not wash it. Just wipe the surface lightly with a tightly squeezed wet paper towel to remove any dirt. The white spots on the surface is umami which seeped out from inside. They are not mold or dirt, and shouldn’t be removed.
Kombu can be found at Japanese grocery stores, Asian grocery stores, or possibly at health food stores. But if you want to order online, here are some of the links;