Riku's Story:
Beat the game with Sora.

Mushu's Summon Card:
Can be found in one of the random doors that appear after the Golden Bar card is used.

Reverse Rebirth mode:
Beat the game.

Sora's Ultima Weapon:
For sale at Mog shops after you've beaten Reverse Rebirth mode.

Special thanks to Revolution readers Saito G, Yukikaze, Randy, SteamPunk and Kim!