Karn, the Great Creator - what defines a 'card from outside the game' in sealed?
It's the same wording as used on e.g. the Wish cycle from Judgment. An example is Golden Wish. Its rulings at the bottom say

10/1/2009     In a sanctioned event, a card that’s “outside the game” is one that’s in your sideboard. In an unsanctioned event, you may choose any card from your collection.

So in a Sealed event, you're limited to your sideboard, which according to the Tournament Rules are the 'rest' of the opened cards you're not using in your deck. (Of course, exiled cards can be chosen as well.)

7.1 Deck Construction Restrictions
Limited decks must contain a minimum of forty cards. There is no maximum deck size. Any drafted or opened cards not used in a player’s Limited deck function as their sideboard.