Perhaps a list will help you out.
It's found in Twilight Town, in the room where Donald and Goofy were sleeping, as Sora. Can only be obtained by Sora only, not Roxas on the final time you visit Twilight Town, before you had into the world that never was.
The ultimate weapon synthesis:
7 Orichalcum+
1 Orichalcum
1 Mythril Crystal
1 Dense Crystal
1 Twilight Crystal
4 Serenity Crystal
1 Energy Crystal
If you need a list on who has them or where to find the items, just ask.
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