Higeta Honzen Soy Sauce.
I have bought this for years at William Sonoma. I have never been able to buy it online, though. I live in fear that they will discontinue it. Every time I go to the Williams Sonoma store I get nervous that they have stopped carrying it – I buy several bottles at a time. A few years ago, I couldn’t find it and I started buying all sorts of different soy sauces to try and find a substitute. I was completely unsuccessful. I noticed today that they are selling a different soy sauce online – I really REALLY hope that doesn’t mean what I think it does . . .
An online search did find two sources that seem to carry it – this one looks less dubious.
This is really the best soy sauce you have ever tasted. You need to keep it in the fridge after you open it.
Try not to use it for cooking, although it is hard to resist. I keep a supermarket brand when I need to use more than 1/4 cup or so at a time. Higeta Honzen Soy Sauce is best savored for sushi or other dipping sauces. I do use it to cook my Asian Sugar Snap Peas, though.