Normally they can be purchased from a gas fire showroom/supplier. You will need to have the make of your fire though as they do differ. Most can be ordered.
As Armada says, you must use only the coals that are specific to that gas fire. Make sure you place them in the exact location as per the manufacturer's instructions as displaced ones can lead to incomplete combustion and carbon monoxide. Manufacturers are now leaning towards coal effect fires that have fixed coals because of the issues arising from displaced coals.