Ancho chili powder is the go-to chili powder for authentic Mexican cooking. It's commonly available and distributed by , among other brands. Look for it where all the other spices are at the grocery store.
If you don't want to buy or can't find ancho chili powder, substitute regular chili powder and a little crushed red pepper. Ancho chili powder is not very spicy, so you don't need much crushed red pepper to match the heat.
For a recipe that calls for 1 teaspoon ancho chili powder, substitute 1 teaspoon regular chili powder and 1/8-1/4 teaspoon crushed red pepper.