You can find fresh or frozen udon noodles at East Asian grocery stores in the refrigerated or freezer section. They typically come in packets or bundles as shown below. Add them to a pot of boiling water like this and use a chopstick to loosen them from their bundle. They typically only take 2 minutes to cook. Then drain and rinse under cool water (you can use them hot in other dishes, but this is a cold/room temperature noodle salad).