Kikkoman makes a very good instant miso soup if all you want to do is the hot water thing. I agree with the above poster about it being much better if you make it yourself. Just stop by an Asian foods shop and get some dried dashi (fish stock) and some miso paste out of their fridge. Dissolve the dashi powder in boiling water, add a couple teaspoons of miso and you've got miso soup. Add some dried seaweed and some tofu chunks if you want.