Help content comes from multiple locations, and is dependent on the modules you currently have installed on your system, it's not from a single source. PowerShell handles the inventorying of version information and help uri for each module and applies logic to determine if the help for each specific module requires to be updated.
You can use Save-Help to store current help files offline, which you can store centrally. Then you just need to use Update-Help with the -SourcePath for any systems that you want to update the help on.
Keep in mind that the help content downloaded from the internet is linked to what modules you have on your system, so try and always use a server that has the most complete set of modules on your system.
Help can also be linked to the revision of PowerShell installed, so if you're using a mix of Powershell 3 and 4 systems, you'll want to ensure you have all the help files you require.