Updated for 2020.
Some links to download Microsoft Visio stencils of the most popular vendors.
Dell Requires registration
Checkpoint happen not to have official stencils set, only Nokia appliances stuff can be found. So someone volunteered and using icons/press releases/PowerPoint presentations done by the Checkpoint turned it into the Visio stencils:
Additional place to look is Checkpoint.com which actually redirects to Netxoom company