During "Bringing Down the Sky" after fully completing the mission ask for the Heavy Armor reward.
Loot it as a random drop from enemies, or inside item containers(rarely appears).
Purchase a "Kassa Fabrications" liscense from the C-Sec armory. It may appear rarely in the Normandy's armory for purchase.
Purchase it from a normal merchant such as the C-Sec armory(rarely appears).