QUARIAN armor is rare. You can find it rarely from defeated enemies or inside item containers. It will rarely appear in the inventory of merchants, if given the option to browse multiple sections of items choose "Non-Human Armor" to filter your results.
If you complete "Bringing Down the Sky" DLC mission, you can ask for QUARIAN Armor as a reward for finishing it.
If you complete "Pinnacle Station" DLC mission, you can buy random items from a desktop computer. Some of these random items will be QUARIAN Armor, including a chance to get QUARIAN Colossus X armor from the 220,000 credit option.