Sake (酒, さけ) should be available to you if you go to a liquor store. Even if they don’t have a lot, they should have at least one brand. My favorite brand for cooking with is Gekkeikan, but as long as it isn’t a flavored sake, any brand will do!
If you can’t get sake, you can substitute mirin and add less sugar if there is sugar in the recipe. If you can’t get mirin or sake, you can try a dry sherry or white wine. If you can’t use alcohol, the next best thing would be rice vinegar, but be warned, vinegar will make the dish taste more sour, so don’t use too much of it or you will end up with a completely different taste!Buy on Amazon