I follow U.S. Supreme Court rulings ...

• oyez.org [oyez.org]
• SCOTUSblog [scotusblog.com]
• Supreme Court Review [supremecourtreview.com]
• Federalist Society's SCOTUScast [fed-soc.org]

I know who Manchester United is - but who the heck is "Citizen"?

Official source: http://www.supremecourt.gov/opinions/obtainopinions.aspx [supremecourt.gov]

However, the official source suggests some unofficial sources, and one of them is http://www.oyez.org/cases [oyez.org], that I listed in my original post.

oyez.org is a very good resource. They briefly summarize every Supreme Court case and decision. There are links to the full written opinion, oral argument transcripts, an audio player with oral argument audio synced with the transcript, and audio of the oral opinion announcements when available.