The bolt is stepped and has a larger diameter for the shock bushing and a smaller threaded portion.  It prevents you from overtightenign the nut and putting the shock in a bind.

You will most likely have to order one....or get a used one from somebody.

You could probably use a large bolt and a large nut from a hardware store temporarily - but you are not going to like the way it looks.  Use a nut with a nylon locking insert.

What size bolt would I need if I got from a hardware store?

9/16 inch stateside, M14 in the civilized world...