After reading an article about Takara mirin and why you'd want to cook with it in the NY Times a year or two back, I decided to try it out. I wandered into a liquor store (in NY, its alcohol content requires it be sold in liquor stores) that just happened to stock it and found that it does indeed make a difference most of the time that mirin us called for.
Now, my bottle has only a few teaspoons left and I can't remember where I purchased it. Ideally, I'd love to find some in upper Manhattan (Inwood, Wash. Hgts.) or nearby Bronx, but mid- or downtown would work, too.
Earlier threads on mirin seem to indicate some asian food marts carrying it, but I doubt that it could be the real thing unless they have a liquor store nearby.