Miso soup is very easy to make. Ingredients can be many kinds of vegetables; sliced white turnips, chopped turnip greens, juliennedi icicle radish, chopped icicle radish greens, onions, tofu, mung beans sprouts, wakame (seaweeds), eggplants, potatoes, spinach, shiitake mushrooms and so on. An egg can be dropped as well.
As you need only small amount of ingredients, people tend to not to buy something particularly for miso soup, but use whatever they have at hand. It is common to paring ingredients, such as turnip with turnip greens and tofu with wakame (seaweed), so that the soup looks colorful, tastier, and nutritionally more balanced.
In Japan, greens of white turnips and icicle radish (daikon) with their roots are common ingredients of miso soup. In the United States, it may not be easy to find them. I experimented to use radish greens for miso soup to substitute. They work well. You can experiment your miso soup using American vegetables. I saw one of my Japanese friends used dandelion greens.
If you watch the amount of salt intake or worry about MSG as I do, I strongly recommend to make your own dashi, rather than using granulated dashi mix. You can also make salt and MSG free instant dashi powder easily.