This armor is rare because of how strong it is. You can find it rarely off of defeated enemies or inside item containers. You will need to be a high level to get the grade "X" armor. If you complete "Pinnacle Station" DLC mission, there is a chance you will get QUARIAN Colossus X armor every time you pay the 222,000 credit shipment in the apartment on the planet Intai'ei. You can also get HUMAN MEDIUM Colossus X armor this way.