It's not actually available in the game, but if you have the cheats activated you can cheat it. Two ways:
1. Install Shadowkeeper from
here and simply put it in your inventory. Of course it's ten times as simple just to change Korgan's alignment with it (so that he also can still wear a good other helmet) but maybe you think that's to cheatish (I did it once though, my Viconia, Korgan and Edwin were all chaotic neutral)
2. use the cheat codes.
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Melusine
Your voice is ambrosia