Amano Foods is a Fukuyama, Hiroshima based company that specializes in freeze-dried foods. They have a huge range of miso soups (green onions, aubergine, nameko mushrooms, mixed vegetable, tofu, spinach, and more) that come in a package as a single freeze-dried serving. Just plop the cube into a bowl and add water. None of the messing around squeezing raw miso out of packets, and the taste, probably thanks to all the proper-sized hunks of vegetables in the soup, blows every other instant miso soup I've had out of the water. (ahaha, water. like you use to make soup.)
But, compared to non-freeze-dried competitors, the price on these items is a bit expensive. In local supermarkets, the soup packets run about 120 yen per packet. But if you try a couple and like them as much as I do, you might want to order the soup online instead of buying locally. On the official Amano Foods online shop, you can get any flavor they make in a box of ten (starting around 1100 yen per box, or 10 of a smaller, "ひとくち" mini-sized cube for 900 yen). You can also mix and match 10 flavors you want into one box at a bit more cost.
In addition to miso soup, Amano also makes freeze dried curries, noodles, donburi, and ochazuke packets. These run about 20 grams and 300 yen each (on the official site).
For the best prices, though, check out Amazon. They offer almost as wide a selection as Amano's official site, including my personal favorites: aubergine, green onion, and butajiru (pork and miso soup). The other perk of Amazon is its free shipping, even on a single box. Amano's official site will charge 500 yen for shipping unless you order at least 5,000 yen worth of product, although thanks to the freeze drying, the shelf life on these products won't pose a problem if you decide to stock up.