That's a mercury part, and any merc dealer should be able to get it for you.
That cable is the steering assist cable. Personally... I would just leave it dangle. It's easier to add throttle as you need it, over letting the steering system crash you into a dock. That cable has been disconnected from mine since the day I bought the boat.
******EDIT****
I just did a quick search, and it's listed as "Obsolete". SO... the part numbers were changed to NA. You may have to make a trip to a dealer to find the part number. Besides... a small shop may have those parts on the shelf.