My wonderful butcher saves his beef bones for me. I went in the other day and he pulled out the whole leg of a cow! I just had the thigh bone trimmed into cavie sized pieces and one large knuckle bone (the knee bit, I guess??) and off we popped. He only asked for £1 to go in the charity box, so cheaper than pet stores too!
You can ususally get all sorts but it is worth phoning coz sometimes, with all the boneless meat they sell now, they don't have any. My butcher will get boned beef in especially for me if I want the marrow bone! They freeze well to!
Obviously, these sorts of bones are just recreational, they wont eat them all because beef leg bones are too tough.