when you battle yami yugi enough times we will trade monster reborn for you cathedral of nobles

There's a special mode on your duel disk.

1. Access your card list

2. This will show the card packets which you've unlocked.

3. Check each one.
The cards that are face-up are the ones you own.
The cards face-down... well, you get the idea.

4. Move your crusor over the face-down cards.
This should tell you the cards from the certain pack(s) you haven't found.

5. If you still can't find it, you should level up to unlock more packs.