Antique Coins are used to purchase three upgrades, available (initially) from three cages hanging in the trailer inside The Yard. Much later in the game, these cages appear as well in the hut in the Swamp. If you have purchased upgrades before, those cages will remain empty there as well. The upgrades are Steroids (3 Coins), Stabilizer (5 Coins) and the Magnum (9 Coins).
#1. Inside the Guest House, Kitchen. Play the Derelict House Tape and at the start, take a few steps back to find a lockpick. Use this to open the locked drawer in the kitchen. Back in reality, open up the drawer to find this Antique Coin.
#2. Main House, 1F, Floor Hatch passage. Go through the floor hatch beyond the kitchen and check out the lawnmower in the passageway underneath the house. This Antique Coin is laying on it.
#3. Main House, 1F, Main Hall. Inside a drawer in the main hall.
#4. Main House, 2F, Recreation Room. Check out the corners of this room.
#5. Main House, 2F, inside the Bathroom. Check the toilet for this coin.
#6. Main House, 1F, inside the room adjacent to the Drawing Room. Check out the ash tray next to the lantern on the table.
#7. Main House, B1, Processing Area. After pulling open the green divider, check out the left wall just beyond. Crouch and check behind the wooden plank to find this coin nearby.
#8. Yard. Down the steps of the Main House, go right immediately to find this coin inside a small pot.
#9. Yard, inside the Trailer. Simply next to the bird cage.
#10. Old House, 1F, inside the Cellar. Check out the tray table just outside the Cellar room to find this coin lying on top of it.
#11. Old House, 1F, Safe Room outside. Check out the Safe House beyond the back of the Old House. Look on the toilet.
#12. Old House, 1F, Altar. Check out the drawer in the corner.
#13. Testing Area, 1F, room with bombs before the Barn. At the end of the long hallway leading to the Barn, enter the door in the corner. Clear the bombs here and check the green desk drawer to find this coin.
#14. Testing Area, 2F, Area after finding the burnt corpse with note. Go through the next door, but before heading downstairs, check out the plank in front of you. This coin is on top.
#15. Wrecked Ship, S1, Ventilation Shaft. This is an easily missed coin. The first ventilation shaft you need to crawl through has a dead-end on the first right. Check it out to find this coin.
#16. Wrecked Ship, flooded area where you drop down into. Before going up the next ladder, check the left side of the area. You need to be crouched and the coin's 'pickup' command will appear only then. On the picture it's on the left side. The coin is easily missable.
#17. Wrecked Ship, 3F, Control Room. Open the locked orange box with a lockpick to find this coin inside.
#18. Wrecked Ship, S2. Use a Corrosive on the locked box on the wall, found between the two Engine Rooms.