From Memory Alpha:
The location of Memory Alpha is shown in an illegible part of the large star chart graphic created for TNG: "Conspiracy". In the chart it appears to be near Sol and Sirius.
In "The Lights of Zetar", Memory Alpha was destroyed. Admiral William Thomas Riker 22:00, July 26, 2011 (UTC)
It's on that map, Alpha Centurai sector block, quite a way above the Sol system (Earth Space Dock) and above Alpha Centurai system.
Star Trek Online isn't canon. But I guess it's as good a map as any.