where can I get the real media
I can see it's still available via my visual studio subscription. You could also check the volume center for downloads. There never was a public download (microsoft.com) for Server 2008 R2 licensed media.
I was wondering if it is possible to use my existing genuine product key I successfully installed for Windows 2003 R2
No, this wouldn't work. You'll need a valid 2008 R2 product key in order to activate.
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