i just read Sora The Key's card guide. id strongly recommend opening some chests, whether it may be False Bounty, Calm Bounty, or Guarded Trove. Normally, the chests reveal Battle Cards (Attack Cards). Also, the Key to Rewards will also give you some super cool Battle Card you wouldn't normally get in just any old room. Other Reward Rooms will give you sleights and special abilities, but sleights are just one part of bonuses you need to care about.
For Enemy Cards, mostly try the Teeming Darkness and the Looming Darkness. Those drop more frequently, and ive noticed. So when you get the cards, hold onto them! if you're in a Roulette Room, get a few of those cards if you want. Trust me, its difficult, but you'll get the cards one baby step at a time. im still in the process- bc i beat Marluxia already -of getting them all so i can get those Gold and Platinum cards! In case you didn't know- not that im trying to make you feel stupid -, you can get some Enemy Cards from beating their Battle Boss counterpart.
I mean like
Battle Boss Jafar: Jafar Enemy Card
Battle Boss Hades: Hades Enemy Card
Battle Boss Maleficent: Maleficent Enemy Card
Battle Boss Riku (Replica): Riku Enemy Card
Battle Boss Vexen: Vexen Enemy Card
Battle Boss Larxene: Larxene Enemy Card
Battle Boss Axel: Axel Enemy Card
Battle Boss Marluxia: Marluxia Enemy Card
Defeat the corresponding Battle Boss to its corresponding Battle Boss Enemy Card. But you probably knew that. Also, in Sora The Key's guide, it says that if you complete Sora's Story, you can go back to the Key to Rewards rooms and aqcuire them in the chests. id try that, if i were you.