Beef Bones can come from all parts of the beast, but those from the shank (photo specimen) are particularly good because they include some of the most flavorful meat and plenty of connective tissue. These are also very easy for the butcher to cut up on his bandsaw. In any case, what you're looking for is bones with some meat and tendons on them, and cut or broken to expose marrow in the center.
More on Beef Shank
Buying: Any meat market should be able to provide plenty of beef soup bones. If they aren't on display ask the butcher. Most markets will cut up large bones for you on the bandsaw. Be sure to specify you want the bones for soup stock, not for your dog.
Prep: Wash well to remove saw kerf residue and bone chips. If large, you should split them into pieces. Always split lengthwise as bone is very hard to split crosswise. Unless you have a lot of practice swinging a meat cleaver, place the edge where you want the split and hit the back of the cleaver with a heavy soft faced mallet. This will minimize pieces of bone flying all over everywhere.
Cooking: These are used to make very good beef stock. Beef stock needs to be simmered a very long time, 8 hours is definitely not too much. Put vegetables in for only the last hour.