Armor and Stealth suits:
Before entering the train car at Armory Station,
you can purchase a Stealth Suit for 100 military cartridges
from the vendor across from the Ammo Exchange. There is a
vendor next to that one that sells an Armor Suit that decreases damage.
Both suits an be worn simultaneously. A similar Armor Suit
is also sold by a vendor next to the Ammo Exchange in Polis.
In addition to buying the Sneaking Suit at the Armory Station,
you can get a pair of nightvision goggles at Front Lines.
While in control of Artyom, turn around, and follow the train tracks.
The nightvision goggles will be in a red breaker box guarded
by two soldiers.