The best substitute for masa harina is ground-up corn tortillas. Masa harina, a finely ground flour made from corn, is treated with a solution of lime and water. It is often used to make corn tortillas as well as many other popular Mexican dishes.
Because masa harina is soaked in lime, normal cornmeal or polenta is not a good substitute for this corn flour. It is also not advisable to use wheat flour as a substitute. In the absence of masa harina, the best thing to do is to place four or five corn tortillas in the food processor and grind them finely. This is the only way to ensure that authentic lime flavor.