You can buy fresh shitake mushrooms in English supermarkets or Asian food shops. Leave the mushroom in a very cool and dry place in the house for 5 days to a week. The mushrooms will completely dry out. Keep them in the container.
Shitake mushroom stock "dashi"
Put the water and dried-shitake mushrooms in a deep pot and soak for 30 minutes to 1 hour. Or if you are short of time, you can add hot water for 10 minutes. Squeeze out the mushrooms and then remove them from the stock. You can use the mushrooms for another dish.
Try making Shitake mushiroom Miso soup!