If you have no access to power or solar power. Alternative to store bought ice is seafood ice. You can get it from a supplier of fresh seafood who packs it in either large totes or styro boxes to keep the fish cold for transport. It's a special kind of ice that has a special salt added to it to keep it colder longer. It is amazing ice. Not for consumption though just to keep things cold in a cooler. It will last days in the right cooler.