There are various dashi substitutes on the market, but this key Japanese ingredient has quite a distinct flavor that is not always easy to mimic.
A staple of Japanese cuisine, dashi is a simple and delicious ingredient that is little known outside Japanese cooking. Dashi is made from water and a salty base ingredient of dried kelp (kambu), fish flakes, anchovies, shiitake mushrooms or dried sardines. It is featured as a stock in soups, broths, and noodle dishes, and can add rich flavor and depth to a dish. The word dashi literally means ‘stock’. However, if you can’t find a commercial, powdered dashi, it is possible to make your own at home with white fish or shellfish, kombu powder and mushrooms, and broth or stock powder. 
White Fish or Shellfish
To make a substitute that tastes the most like commercial dashi, try using white fish or shellfish as a base flavor. Mild white fish flavors work well for dashi, such as cod, bass or halibut. For both white fish and shellfish, the parts of the fish that work best are the tail, head, and skins.
Be sure to wash the parts well before use. You can use a pre-prepared stock or make your own by sautéing a range of vegetables, including onions and garlic. Once you have a thick liquid stock, add a bit of white wine and your fish scraps. Simmer for half an hour to an hour, and strain if needed. Feel free to add some sugar or soy sauce to your dashi, but bear in mind that this is a type of stock and not a soup in and of itself. 
Kombu Powder and Mushrooms
Another traditional Japanese recipe uses kombu seaweed and shiitake mushrooms to create a deep, salty, and earthy flavor. It will probably be easiest to find dried versions of both kombu and shitake mushrooms, which will work fine – simply follow the soaking instructions to get them wet and ready for use. If you can’t find kombu seaweed, any other kind of dried seaweed will suffice. Soak the mushrooms and seaweed for approximately half an hour, then bring them to a boil and simmer for five to ten minutes. Add a bit of seasoning and stock to taste. 
Broth or Stock Powder
This works as a good emergency substitute since dashi is essentially a Japanese stock liquid. While these options may lack the desired sea flavors, a regular broth or stock powder does make a quick and easy alternative. Pre-made chicken, fish or vegetable stocks are your best bet, as they will provide a simple and mild background taste.