I have seen two kinds of mirin in the supermarket. However, for people like me who prefer not to consume alcohol even in cooking, it's necessary to know these two kinds properly.
My husband explains that mirin is simply just sweet Japanese sake. That's the original type, called hon mirin. You'll usually see the characters 本みりん on the bottles of this kind of mirin.
On the other hand, the one I use has alcohol content as much as those in non-alcoholic beers. And I assume that the 0.003% or whatever invisible amount of alcohol in there is essentially evaporated when you cook it. This type of mirin is called みりん風調味料 or in alphabet, mirin-fū choumiryou. It literally means "mirin-like seasoning". Some translate it to "low-alcohol mirin".
If you don't have access to any Japanese store near you or if online shopping is not convenient in your place, perhaps you can try using diluted honey or maple syrup or Sprite in recipes needing this.
If you have mirin but you don't know how to use it, check out my recipes with mirin by clicking mirin on my tag cloud or by going to this link.