Was it in Chicago? Italian Beef is a No. Illinois/ Chicago thing. It is very doubtful that you would find it in a grocery store. You have to make your own!
My mom passed on her great Italian Beef recipe to me-- it is a throw stuff in the crock pot and cook until it is done recipe, no formal instructions or amounts.
1 chuck roast (like you would use for pot roast) as big as you need
vegetable oil
1 jar pepperoncini peppers
beef broth, a can or two, or three
Liptons Onion soup mix
Italian seasoning
Brown the roast in the oil on all sides in a pan on the stove. Transfer to the crock pot. You can deglaze the pan with a little beef broth and add all that if you want. Add beef broth to cover. You can add water if you need a little more liquid. Add the onion soup mix. Pour all of the liquid from the pepperoncini peppers in and a few of the peppers depending on much heat you like. Dump in a bunch of Italian seasoning, I usually use about half the jar, maybe 3/4.
Cover and let it cook for about 8 hours. It should be falling apart. Remove the peppers. Shred beef in the juice and serve on really good quality sub rolls warmed. You can add extra juice to the sandwiches or serve on the side. Add Provolone or mozzarella cheese if you like, and also serve some more of the peppers.
This is really almost better made a day ahead of time and then refrigerated overnight. You can also make it on the stove in a large pot. Cover it and let it cook on low heat for 6-8 hours.