buying additional powerline receiving modules

[Pkrucz]

I have 1 powerline AV500 pair working very well downstairs in our home.   I have a server upstairs that I'd like to wire to the router, and easiest thing to do is to add a 2nd AV500 receiving adapter upstairs.   But, can I buy a separate unit or do I need to buy the pair?

I see it may be EOL too, so the question applies to a new replacement system... can I buy separate pieces?

[netwrks]

Not sure how one powerline adapter will create a LAN segment, from your device back to your router, or switch...

One powerline adpater to your server, then what?