Hello.
My car is leaking coolant through this rubber "plug" (where the red arrow points to) on the sub-tank top. This plug has a hole in the center and coolant just comes out of it when I pressurize the system. I don't see it as a separate part number, so I assume is part of the whole tank assembly. Are there any options other than buying a new tank assembly?

 

Its not leaking... its purging. It's doing exactly what it's supposed to do. You're doing the pressure test wrong if it's making into the overflow. Reassess your method