Scott,
Try a small chuck roast or other fatty cut of roast. Ask your butcher if in doubt. You can also cut a store-bought roast in half and freeze the other half for future use.
Also, keep in mind, this stuff freezes VERY well after it has been cooked and can be used in so many other dishes. My freezer has a constant supply of shredded pork and beef that gets used almost every week in some other dish.
Mexican dishes especially are great for leftover pork and beef. Also, you can substitute shredded beef for ground beef in many recipes.