Hey jinty! Any big ferritera in Santiago Like Ochoa, will provide you with a plastic water tank of the size you need and most will get it installed for you. Unless you own the building, you will have to get permission from the owner to get it put on the roof. The installation may require strengthening of the roof.
Because the valves sometimes stick open, (ie overfilling the tank), be sure you know exactly where the water shutoff valve is! Good luck.. Canadian Bob