Quarian armor is very rare. It can rarely be found off of defeated enemies, inside item containers or from armor merchants.
The only way to guarantee you can find quarian armor is to complete the DLC missions: "Bring Down the Sky" and "Pinnacle Station" You can ask for quarian armor as a reward for completing Bring Down the Sky. After finishing Pinnacle Station you can spend credits on random item shipments, each shipment has a chance to have quarian armor. The 220,000 credit option is the most reliable