...but it is helpful to know you can get unlimited replacements for any card, no matter how rare.

Basically, you just need to find the weakest opponent on board the Ragnarok (whoever's using the simplest rules, basically) and keep playing him or her until every rare card is back in your hands. Any given CC Group member can theoretically have any replacement card; a card doesn't get 'assisgned' to a member until you actually play that member and see the card in their deck.