Skirt steak should be labelled as “skirt steak” at the grocery store. However, it may sometimes be called “fajita steak”. It’s often sold as a long, thin cut of meat, but it may be pre-sliced ready for cooking. Flank steak is sometimes referred to as “London Broil”, although this is technically the name of a dish, rather than a cut.
It isn’t unusual to find skirt or flank steak at a grocery store, but they aren’t stocked everywhere.
The best place to buy specific cuts of meat is at the butchers. As they tend to receive the entire animal, it’s easier to get large cuts of the leaner meat. If ordering skirt steak, you may be asked if you prefer “inside skirt” or “outside skirt”. Truthfully, there isn’t much of a difference. Outside is thicker, and inside shrinks more when cooking.