It can only be found in four different locations, and two victims need them, so the player should conserve the Salts for either the Victims needing them or for a very tight situation with Extreme Effort.
The first can be found in the Skyscraper episode, on the stone platform above Victim #3 (the man asking whose hand is under the rubble and needs to be amputated). Can be accessed on the way down, after the player has located the medicine crate. The second can be found in the north-eastern room of Henry's apartment in Episode 9.
The fourth can be found above and a little to the west from Victim #8 (the man who asks cigarettes on the wooden ramp by a building outside Henry's apartment). Jump on the next wooden ramp and then climb onto the crate and on the roof to locate some Painkillers and Salts. And the fifth can be found on the 2nd floor of the Blue Hotel as shown by the image above. A bullet is needed to shoot off a lock on the door leading to this area.