If sour orange juice is not available, use a combination of regular orange juice and lime juice; for the 1 cup called for in this recipe, 2/3 cup orange juice and 1/3 cup lime juice is a good substitution. Jars of fruity, mildly spicy aji amarillo paste are sold in some larger supermarkets and in grocery stores catering to Peruvian or South American customers, or find this ingredient online.