The original SuperNova chuck did not have a hex key, it was a geared key. (The SuperNova2 has a hex key.)
It looks like replacements are not available from Teknatool --
LINK to an FAQ that discusses this and the other 2 geared chucks they make (and the fact that the handles are not interchangeable.)
Down the bottom of that page (follow the link!) it says that a matching key is -- or was -- available from a company in New Zealand .
You might be able to find one on eBay, but it's probably better just to buy a chuck for which the key is readily available.