Pasilla negro chiles are long, slender, dark dried chiles typically grown in Mexico. Mexican Pasilla chiles have an earthy flavor and are used in mole sauce as well as other Mexican sauces and stews (chile negro). In California, the ancho chile is called pasilla, but they are two distinctly different chiles. In Oaxaca, the pasilla negro is smoked, and it is called Pasilla de Oaxaca.
In addition to whole chiles, you can buy ground pasilla negro powder.
If you have access to a Mexican market, you should be able to find whole or ground chiles. You can also buy them online here at Gourmetsleuth.com.
If you don't have pasilla negro chiles you can substitute equal amounts one of these chiles: