Flank Steak should be found in Grocery Stores under the name Flank Steak as I've seen it called before at some stores.
However if your Grocery store doesn't sell Flank Steak as Flank Steak then another name for Flank Steak is London Broil.
So to get Flank Steak at a Grocery Store you'd look for the name London Broil if you can't find it under Flank Steak.