Mirin is a Japanese cooking rice wine, widely used in marinade and sauces. You can find mirin at Oriental/Asian groceries or supermarket Asian aisle.
Highly recommend to add mirin for this recipe but if you cannot find it, you can substitute with dry sherry, or rice wine vinegar.
If you wish to make halal or non-alcohol version, you can mix sugar syrup/honey + rice vinegar/water. ( ratio is ½ + ½ ) Taste won't be exactly same with mirin but these substitute options can give the nearest result.
Soy sauce: Japanese soy sauce is mostly use for this recipe but if you cannot find Japanese soy sauce brand, you can use any light soy sauce available at your local store. ( I usually use Kikkoman all purpose soy sauce. )
Sake: Sake is a fermented Japanese beverage made from rice. Sake is used in meat marinate and sauces in Japanese cooking. It has sweet and acidic flavour and higher alcohol content than mirin. You can mostly buy sake bottles or cooking sake at Oriental/Asian supermarkets. To make the dish more authentic way of cooking, you can add sake. If you don't wish to add sake, you can omit it or substitute with water for this recipe.