Beef bung casing is derived from the cow's appendix, and is purchased whole. It is roughly 6 inches in diameter and 4 feet in length. It naturally has a pungent odor, and is used to case larger cured meats such as coppa and mortadella.
You may or may not be able to purchase beef bung casing from your local butcher. However, you can purchase it from Butcher-Packer.com.