radiator repair kit or some JB weld...from autozone or any parts store but it should be used only as a temp fix, any exhaust shop should be able to repair it aswell.
my rad is a brass one and the leak is on the bottom left right on one of the fins.... the hole is only about the size of a pin maybe a little bit bigger.
well if its only the size of a pin then i'd JB weld it ur gonna have to remove the rad and clean the area where the hole is with some medium grit sandpaper(if possible) then jus slap a lil bit JB on it and let it cure for acouple hours.