I Need One of These... It Leaks!!

I need to get one of these, I'm not sure if it's called either a "Heater Valve" or a "Heater Bypass Valve". My bad I should know... However It's starting to drip a little, seep, and hiss at the arm. So can anyone tell me who sells them or where to get one, that won't leak if its an NOS old one... Seems like I can find everything that fits someone elses but my own. Unless the design has been changed? It's for a 73 duster (that had air) 318..
Thanks everyone for any input..

19....