You go to the store to buy cereal for your breakfast, but you can't find your favorite brand, so you buy english muffins instead. this is an example of

substitute product.

A substitute product, in economics and in consumer theory, is a product or service that a consumer sees as the same or similar to another product. If you don't have the cereal you can find a substitute for the same situation, breakfast,  like another brand of cereal or even something a little more apart like a muffin