This is pretty easy to get. The earliest in the game you can get it is after Priority: Palaven, unless you preordered the game. If you did, once you are able to explore the ship, check your private terminal. Then buy the second one in the Shuttle Bay store.
Armors can be found in the shops throughout the Citadel. There are five armors that you can buy in game, but getting any two will pop the trophy.
The Blood Dragon Armor is found at Kanala Exports on the Presidium Commons.
The Cerberus Armor is found at Elkoss Combine Arsenal Supplies on the Presidium Commons.
The Collector Armor is found at Nos Astra Sporting Goods on the Presidium Commons.
The Inferno Armor is found at Kassa Fabrication on the Presidium Commons.
The Terminus Armor can be found at Batarian State Arms at Docks: Holding Area.
The preorder armors are N7 Defenders Armor and Reckoner Knight Armor.
*There are three other armors (you can only choose one of them for your game) that was released in the Citadel DLC. I do not know if it works in popping the trophy.