An ink supply unit (430) including at least one ink storage chamber (521) for holding ink for supply to a portable ink jet printing arrangement, the ink supply unit (430) including a series of spaced apart baffles (441-443) configured so as to reduce the acceleration of the ink within the unit as may be induced by the movement of the portable printer, whilst allowing for flows of ink to the printing arrangement in response to active demand therefrom. Preferably there are several chambers (521) for holding different color inks and are desirably formed through the injection molding of at least two separate parts which are preferably sealed together to form the ink supply unit (430).