While traditionally the dish is made with freshly steamed thin rice paper pastry, store-bought fresh rice noodle sheets are just as good. They are available from the refrigerated section of most Asian grocers. Dried shiitake mushrooms can be substituted with about one-and-a-half cups of finely sliced button mushrooms if you prefer. If you can't find rice noodles, serve the mixture in lettuce leaves. Baby cos (romaine) leaves are perfect for this.