Here's this info I found might help.(I think someone already pointed this out already but here anyways)
1. Clear Atlantica.
2. Clear 100 Acre Wood.
3. Find the chest in the World That Never Was at the Brink Of Despair just before you go into the castle. There will be a big chest next to the save point. This is not the Orichalcum+. Turn around at this location and follow the short path across from the save point with the Orichalcum+ chest.
4. Clear Goddess of Fate Cup in the Underworld Coliseum.
5. Chest in Twilight Town, in Sunset Terrace where you went earlier in the game for the Seven Wonders. Jump on top of the train and you will see a chest on the roof of the big building (the one closest to to the sunset station).
6. Chest in Space Paranoids during the second visit there, just before you fight Commander Sark and the MCP.
7. Collect all Material in the game and give it to the Synthesis Moogle. He will give you an Orichalcum+. You need every single Material (shards, stones, gems, and crystals, including a regular Orichalcum).
Once you have all the orichalcum+ and other needs just follow this.
Sora's Ultima Weapon is obtained through item synthesis. Once you have obtained seven Orichalcum+s and the other ingredients, you will be able to synthesize it if you have an Energy Crystal. Note: You must be at least a level 2 Moogle to use energy items (for example, shards, stones, gems, and crystals). The items required are as follows: