Go to a military surplus type store, that sells used/vintage/collectible military gear, and find a military issue bayonnete. Not the cheapie Taiwan look-alikes, but the real ones. Any of them from WWII to the present are great digging tools. You can stand on them, and/or pry on them with all your might, and will not be able to bend or break them. You can also maybe find them on ebay, but beware of after-market cheapies. It's got to be the military grade types.
Final note: The ends are made with a butt-thingie that is made to fit on the end of a gun (depending on the type you get, and what gun it was made to fit on). If it is the irritating flat-end type, you can just put a bunch of black-tape wrappings to make a rounded end, no problem.