Mirin is a golden colored sweet wine made distilled sake and steamed glutinous rice. This is a common ingredient in Japanese cooking used in sauces and glazes. True mirin is called hon-mirin and has an alcohol content of 13- 14%. There is also a product referred to as minin-fumi which is a synthetic flavoring with a 1% alcohol content. Mirin has a slight nutty flavor, similar to sherry, which happens to be a reasonable substitute.
Mirin can be found in Asian grocery stores as well as many well-stocked grocery stores including Whole Foods Market. You can also purchase it online at Amazon.com: Mirin.
If you don't have mirin here are the best substitutes: