Visio stencil with shapes for creating C4 Model diagrams.
It comes in two variants: blue and blue/green (thanks to rabidgremlin)
To download, click the .vssx file of the wanted variant in the file list above, and on the next page click 'Download' button.
To install in Visio, normal place is \Documents\My Shapes. If this doesn't work for you, please google how to install stencils for your particular version of Visio and Windows.
Update: Now there's an install script (thanks to nuno-barreiro). It will look up the Visio shapes folder in registry and copy the .vssx files to that folder.
Example of container diagram: