New Bleeder valves are available at most hardware stores. Not just the screw, but the entire valve. The entire valve unscrews with a wrench. You will need to shut off the boiler and partially drain the water from the system so that no water comes out when you unscrew the valve. Wrap teflon tape around the new valve and screw back into place.
You will then have to refill the water to the correct pressure and bleed all radiators in your house.