Personally I would go right to the "horse's mouth" for Mapsource. It was discontinued almost 3 years ago and this was the final version (6.16.3). If you want older versions, then use the link Terry posted. http://www8.garmin.com/support/download_details.jsp?id=209
Probably doesn't matter much either way, but when possible I prefer using the Garmin site.
Now, before installing anything you might want to use the GMTK backup procedure described here. That should let you move all your maps with a minimum of trouble. http://www.javawa.nl/gmtk_en.html
New PC? Use JaVaWa GMTK first, then install MapSource
When you use JaVaWa GMTK to transfer maps to a new PC it is higly recommended to do this before installing MapSource. The downloadable version of MapSource is an update version, that refuses to install when no older version exists. JaVaWa GMTK prepares, when necessary, automatically your PC so that MapSource will install properly.