You get to level 22 and beat Yami Marik Then you will have all packs available however some cards you have to trade to get.
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Unfortunately, no previous YGO! DS games can link with World Championship 2008, so you cannot get the God Cards from Nightmare Troubadour. I know for a fact that "Slifer the Sky Dragon" was offered as a downloadable card for one day, so perhaps that is the only way of getting the God Cards. yes ghosts exists.
Find Ishizu and get and the Egyptian god card.
Exodia Necross is in the card pack The Forever Ones.
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You buy them using your DP you win in a duel, at the card store!