They will keep forever in a tightly sealed container on the shelf. Clean off any dirt with your fingers before using. Please never soak fresh wild mushrooms in water. Instead, moisten a soft cloth or a paper towel, then gently and carefully clean off any little bits of debris. You can use your fingers just as well for the larger bits.
Mushrooms are delicate and easily bruised, which leads to premature discoloring and softening.
One of the best sources of mail order information on fresh and dried wild mushrooms is from expert edible fungi farmer Hans Johansson. He's a grower and shipper to restaurants and gourmet shops, but will sell to individuals in bulk. He can be reached at P.O. Box 532, Goldens Bridge, NY 10526. This sauce is very quick, very fresh tasting, and absolutely great on pasta. Once you've tasted it you'll never use canned tomatoes again.