I take it you stripped it out of the caliper? You might have to go buy a new caliper for it. Probably pick one up for about $35 from PepBoy's. There will be a ~$12-18 core too.
If its just that you rounded the valve trying to get it off you shopuld be able to get a replacement at any auto parts store. To get the stripped one out just put some vice grips on it and turn it the correct direction.