If you didn't purchase the Sneaking Suit at Armory Station, the earliest you can get a pair of Nightvision Goggles will be in Front Lines.

Once you have control over Artyom, turn around and follow the train tracks. Two enemy soldiers will be next to a red breaker box holding this piece of equipment.

(Note: while you can complete the game without the NVG, there will be several more instances when these items appear in plain sight in later stages).