Just go into a hardware store and ask for a 1/2 boiler drain. Which can be be sweat or thread (male). You'll have to try and melt the solder off the old valve and re-sweat (solder) the new one on. Be mindful that you can't have any water in the pipes or the solder won't take. If you have too much trouble I'd suggest calling a plumber so you don't do more damage than good.
