How to fix this valve

[Apark]

So I have a pool that was built in 2003, and this valve, that adds water to the pool when the water level is low, is leaking. When I opened this valve, water started flowing into the pool, but when I closed it, water kept flowing in. I think that the stopper is old and corroded and would like to replace that as I think it would be a fix. What is the piece that I'm holding in the below picture called and how would I find a new one?

Also added a third picture that shows the overall setup.

Thanks for your ideas and suggestions.

 

[vertcnc]

The parts you are holding in your hand are what i call the stem and packing nut. The whole valve is a gate type valve. If you could find the manufacture name on the body, you may be able to find parts at hardware store or plumbing supply. I think the cheapest and easiest is going to just get a new valve from hardware store, Home Depot, Lowes, etc. Looks like you would have to cut the PVC enable to thread on new valve, so you will also need a couple PVC fittings.

 

[kadavis]

In the long run you might want to replace the valve and what appears to be a vacuum breaker above it with a PVB (pressure vacuum breaker) like this https://cdn3.volusion.com/7j5ob.q34uw/v/vspfiles/photos/WA800M4QT-075-2.jpg?1476367678