Mirin is one of the Japanese condiment resembling sake, but has a much higher sugar content. It can be made at home, by adding rice koji to sochu (unflavored distilled liquor, vodka can be used in the U.S.), and let it sit over 6 month. Over time the mixture will be highly alcoholic and sweet. Most of mirin sold at grocery stores contain a small amount of salt in order to make it undrinkable and avoid alcohol tax.
Sake and mirin are not interchangeable, because they are used for different purposes. The thickness and sweetness of mirin is an important property, which sake doesn’t have, to gives a glossiness to teriyaki. On the other hand sake removes the smell of fish or meat, makes food tender, and add depth to flavor. I think mirin affects to the surface of the food, and sake works to deep inside.
Mirin is sold at Japanese grocery stores. For those who would like to order online, here are the links;