I don't have a subscription, but they are scans of USGS 1:24000 topo maps. These are available free from the USGS here: http://viewer.nationalmap.gov/viewer/
In the Base Data pane on the left side of the screen, check the "Scanned Topo Maps" box and zoom in until the scale shown at the bottom of the screen is 1:24000 or less. Or if you prefer a google maps interface, go here and click the Topo button on the top of the screen: http://mapper.acme.com/
These are raster based maps - scans of paper maps. Garmin's regular 24k topo maps are completely different. They are proprietary vector based maps that feature routable roads.
