Google search is your friend. Lots of threads on various forums on this topic.
Parts are not impossible to find, even if it means upgrading the valve, and patching the new one in after cutting out the old one. Before you go that far, check the valve itself. Some valves have inner portions that can be unscrewed and replaced, without taking out the whole outer valve body.
Having said all that, first you should make absolutely sure the valve is really leaking. Many valves have a pin in the center with a spring attached to it. Make sure the pin is not locked (via turning it) into a position where the spring holds the valve open. You wouldn't be the first person to make this mistake.