Most of the time, you can re-download the current version of macOS via the Mac App Store, and older ones via these links:

However, I’ve run into a situation several times where the Software Update mechanism simply refuses to initiate a download:

Thankfully, macOS installers can be downloaded via Terminal in macOS Catalina. This command will download the most recent version of macOS, depositing it in your Applications folder:

Open Apple Communities Search. Looks like no one’s replied in a while. Question: Q: Download original High Sierra 10.13.0 More Less. Jan 24, 2018 Download Mac OS X 10.13 MacOS High Sierra.ISO – Download High Sierra.DMG installer – MacOS High Sierra VMWare image, Hackintosh, Torrent download – Without using Apple Store ID. Free Download macOS 10.13.6 High Sierra Without App Store Mac device users can get the final macOS 10.13.6 update directly from Apple by going to App Store Software Update option. Or if you don’t have high-speed internet or WiFi connectivity, then you can also Free Download macOS 10.13.6.DMG or.PKG offline installation files available for.

softwareupdate --fetch-full-installer

The softwareupdate command has some neat tricks up its sleeve, as pointed out by Armin Briegel:

The --fetch-full-installer flag has a sub-flag: --full-installer-version which allows you to download a specific version.

Pages For Mac 10.13.6

During my testing in the Catalina beta version I was able to download 10.15, 10.14.6, 10.14.5, and 10.13.6. I was not able to test if 10.13.6 would download the hardware specific build of 10.13.6 for the 2018 MacBook Pro, since I do not have that hardware.

So, to pull 10.13.6 down, you’d use:

softwareupdate --fetch-full-installer --full-installer-version 10.13.6

I wish Apple would just have a support document up with direct downloads for all of this stuff, but this tool is not a bad alternative.

Download Pages For Mac 10.13 64

Update: Don’t miss this documentation from JAMF for more on the subject.