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.
Example of container diagram:
