If you need an alternative to flank steak, skirt steak or flat iron steak makes the best substitutes as their flavor, texture, and price are very similar.
Skirt steak is often confused with flank steak because of their similarities. Skirt steak is a tougher cut of meat, but it has more marbling, making it more flavorful.
Flat iron steak is also known for its rich marbling. While it's still chewy due to a good amount of muscle fiber, it is still more tender than flank steak.
Unlike flank steak, flat iron steak can be cooked to medium, though medium-rare is recommended. It needs some softening up, so a marinade is a good idea.