Cobra Staff Gems are a single item that can be found in Epic Mickey: Power of Illusion.
They can be found in Castle South Hall fairly close to where Mickey rescues Pluto on a platform above. Mickey can use a Platinum Platform sketch to leap up to the area on the upper right, then use the Archer's arrows to gain access to the upper left platform where the Treasure Chest containing them can be found. They are used for The Sorcerer's Oppression quest.