Cactus paddles, both fresh and canned, can be found at any Mexican grocery. Canned ones can be found at many regular supermarkets as well. In Spanish, they are called nopales or nopalitos, which is how they will be probably labeled at a Mexican grocery. And you can substitute 2 tablespoons chili powder for the ancho chiles if you prefer.
Cactus casserole with rice, ancho chiles and cheese | www.homesicktexan.com