I would find a local bike shop and see if they can match it. Wheels manufacturing makes replacement cones I think, most shops will have an assortment. Most shops also pull axles and cones out of old wheels to save for repairs, that is probably what you will need to find. A bike shop with a big bin of old cones/axles.
Quick google search yielded this link to the wheels manufacturing cones, says sovos, joytech etc. I don't know if they sell them individually, they usually come in the assortment box like shown here:
http://www.beyondbikes.com/ItemMatri...mpaign=Froogle