You'll get Ghetsis at N's Castle. When you beat the Elite Four, you can go around with Looker to find the other six.
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Zinzolin is met and arrested in Cold Storage.
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Ryoku is met and arrested in Relic Castle.
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Giallo is met in Dragonspiral Tower and arrested on Route 14.
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Rood is met in Castelia City and arrested on Route 18.
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Bronius is met in Castelia City and arrested in Chargestone Cave.
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Gorm is met in Pinwheel Forest and arrested in Dreamyard.