Coolant Reservoir?

[Engz]

Well I've been getting "Low Coolant Level" warning on and I've been adding coolant here and there and I realize that my coolant reservoir is leaking. The main reservoir's body is in 2 pieces and there should be a gasket between them. I think the gasket leaks. I went on RealOem and looked at this diagram, it does not show a gasket.

Am I wrong about it having a gasket inbetween the 2 pieces or they don't sell that seperately but instead of a whole part. But I don't see the part number for the reservoir.

Does anybody know?
Thanks for any inputs.

Here is the piece I am talking about circled in gray.