i need new brake caliper bleed nipples?

i have changed the front discs and pads on my vauxhall zafira 2005 and am now suffering from spongey brakes. however upon trying to bleed the brakes the bleed nipples both front and rear have rounded off, so i cannot undo them to bleed the system. how do i find out what size threads they are to get them from an altenative supplier as vauxhall will take 3 days to get.