In a sealed chest in the pool at Baaj Temple, hidden in a corner on the opposite side of the pool from the doorway to Anima's shrine. The chest will not appear until you've defeated Geosgaeno, and isn't visible until you're literally right on top of it, so dive deep and keep hitting the [X] button to search for the chest. You MUST have the Celestial Mirror in your inventory to open the chest, and therefore get the weapon.
In a treasure chest that appears during your second visit to the Farplane. You can get this after you've finished everything you need to do in Guadosalam for story purposes.
Dodge the lightning at Thunder Plains 200 consecutive times by pressing [X], then go back to Rin's travel agency and examine the treasure chest out front. Do not save in between dodges or leave the screen because this will reset the dodge counter. Also, don't just sit there and hammer [X], or use a turbo controller, because the game senses you're trying to cheat, and will zap you with lightning just to annoy you. You can get the Sigil on your first visit to Thunder Plains, long before obtaining the airship, unlike everyone else's Sigils.
[Break Damage Limit] + [Triple Overdrive] + [Magic Booster] + [One MP Cost]
Does more damage when Lulu's MP is full, and allows Shiva to exceed 9,999 damage.