Yuffie’s Ultimate Weapon, The Conformer, can be obtained in the Sunken Gelnika. It is in a treasure chest in the first room in the top left corner of the room. This treasure chest cannot be visibly seen so it’s best to bash the [OK] button and you should get it.
Location of the Sunken Gelnika can be found on the World Map if you are unsure of where this is. The damage of this weapon is based on the enemies’ level. The higher the enemy level, the more damage it will do.