Acitron is made from the very large round biznaga cactus (Echinocactus grandis) In Mexico the acitrón would be found in candy stores and sold in the form of square bars. The bars would typically be cut into small cubes and added to a variety of desserts, sweet tamales and even in some picadillo recipes.
Don't confuse acitrón with citron, which is a type of citrus fruit.
Look for acitrón in Mexican markets or Mexican bakeries. This product is becoming increasingly difficult to find even in Mexico because the large barrel cactus is now endangered. People caught harvesting the cactus are subject to very high fines. You are best just using the substitute suggested.