What is miso?Miso is basically a fermented condiment. It's made from soybeans, barley or rice, or a combination thereof. Miso can also be made from chickpeas for a soy-free option. It has a salty taste but a unique one. I really like the sharp taste in "cheese" recipes. It's also great for making broths and dressings and you can use it instead of table salt if you like.
Is there a substitute?Unfortunately, no. Miso is very distinct and there is no substitute for it.
What brand should I buy?Don't worry about brand so much as color. I prefer yellow miso, which is a mellow miso. White and red misos are an okay substitute but steer clear of brown or any dark colored miso. (They are really strong and taste different. Brown misos won't work in HH recipes.)
Where can I find it? Miso is refrigerated, usually by produce and other refrigerated condiments (like dressings). You can find it in health food stores (such as Whole Foods Market) and Asian grocery stores. My little corner street bodega in New York City even had it. Always ask, chances are it is in your store but hiding.