A French dip sandwich, also known as a beef dip, is a hot sandwich consisting of thinly sliced roast beef (or, sometimes, other meats) on a "French roll" or baguette. It is usually served au jus ("with juice"), that is, with beef juice from the cooking process. Beef broth or beef consomme is sometimes substituted. (from wiki)
So cook the beef yourself or by any beef stock - Knorr, Oxo, Bovril, .....