Dried porcini add depth of flavor. While you don’t need such exquisite and pricey specimens for things like soup, know that the better the mushrooms, the better the flavor will be.
Available at many supermarkets as well as specialty markets, their quality can vary. When buying them, look for large pieces that are uniform in color. Darker pieces usually are more intensely flavored. Make sure there are more caps than stems, and avoid those that appear lacy, a sign they may have been infested with insects that vacated the mushroom during drying.
If possible, smell them; the aroma should be deep, rich, and inviting. Some porcini mushrooms have a grade on them; if you see an A (or double or triple A), these will be larger cap pieces.
Keep dried porcini well wrapped in a cool, dark place, where they’ll last for months.