Resident Evil
The chemical mixture is found on the floor in the eastern save room under the stairs to the 2nd floor. From here you'll have to walk over to the greenhouse to use it.
Resident Evil Director's Cut
This time getting finding the chemical just requires to move outside to the boiler behind the bathroom. The chemical is found sitting on the ground.
Resident Evil Remake
The "Herbicide" in the Resident Evil remake is located in the same place as in the Directors Cut, outside the eastern side of the mansion on the 1st floor.