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Several ways.

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).