You can load waypoints/routes/tracks in MapSource for an area where you don't have maps, they will just show on a blank background. So you can still transfer them to and from a GPS device.
If you install Garmin BaseCamp, it includes a "Global Map", which will also be available in MapSource. This map is very lacking in detail, and rather inaccurate, ie it only has A roads. But maybe a bit more useful than a blank map.
You can get free maps from OpenStreetMap. This is the easiest website:
http://garmin.openstreetmap.nl/Just pick a country/state, and it will give you a download link, which includes an installer for MapSource. These maps are usually pretty good, mostly accurate and complete for roads. Though parts of the US may be a bit behind.