Puya chiles are quite similar in appearance (but smaller) and flavor as the guajillo but with a more intense heat. The puya can be used with guajillo to make the dish more "piquant". You can also mix the puya with other mild chiles like New Mexico for a richer, hotter sauce.
Puya are typically found in Mexican markets and less so in larger local grocery stores. You can also buy puya chiles online in our store at Gourmetsleuth.com
If you don't have puya chiles you can use equal amounts (by weight) of: