This is where strict following recipe lovers get frustrated. I don't put specific peppers here because it will depend on what you have available in your geographical area. Most standard recipes call for 3 ancho peppers (dried poblano peppers), 3 cascabel chiles - also known as the little bell, and 3 arbol chiles (or substitute cayenne peppers).
We use whatever we have in our garden that year but you can also buy a variety of dried chile peppers at a local or international market. The key is to have a variety of peppers that produce a variety of flavors and heat.
Recipe courtesy of Old World Garden Farms