The flavour of miso is unique but when really trapped I have sometimes used dried, porcini mushrooms, smashed to smithereens in a mortar, then reconstituted with water. Its different, but it works! It is a good umami paste in general, actually :)

Depending on the amount needed and the character of the recipe, you could use soy sauce. You'll get saltiness and flavor, but it's not the same. If the taste of miso is crucial, put that recipe in your to do pile and miso on your shopping list. And look at the recent foodpickle about different kinds of miso. If this is new to you, read up -- it's a wonderful complex flavor family.

The flavor of miso paste is unique, and there is no substitution for that. If you need a small amount in your recipe you can try using tahini or sesame paste.