Obtaining re-certified macOS installers

Re-certified macOS installers can be obtained using these links:

Mac OS X Yosemite 10.10.5 [1]
https://support.apple.com/HT210717
http://updates-http.cdn-apple.com/2019/cert/061-41343-20191023-02465f92-3ab5-4c92-bfe2-b725447a070d/InstallMacOSX.dmg

Mac OS X El Capitan 10.11.6 [1]
https://support.apple.com/HT206886
http://updates-http.cdn-apple.com/2019/cert/061-41424-20191024-218af9ec-cf50-4516-9011-228c78eda3d2/InstallMacOSX.dmg

macOS Sierra 10.12.6 [1]
https://support.apple.com/HT208202
http://updates-http.cdn-apple.com/2019/cert/061-39476-20191023-48f365f4-0015-4c41-9f44-39d3d2aca067/InstallOS.dmg

macOS High Sierra 10.13.6 [2]
https://support.apple.com/HT208969
https://itunes.apple.com/us/app/macos-high-sierra/id1246284741?ls=1&mt=12

macOS Mojave 10.14.6 [2]
https://support.apple.com/HT210190
https://itunes.apple.com/us/app/macos-mojave/id1398502828?ls=1&mt=12

macOS Catalina 10.15.x (current release) [2]
https://support.apple.com/HT201475
https://itunes.apple.com/us/app/macos-catalina/id1466841314?ls=1&mt=12

ref: https://support.apple.com/HT208052

Notes:
1. The direct-link downloads aren't the actual installer apps, but the installer apps bundled in a .pkg -- run the 'InstallMacOSX.pkg' or 'InstallOS.pkg' and it will put the full 'Install [OSname].app' into your /Applications folder.
2. The App Store direct-access links should work from within any OS on a Mac or VM capable of running that OS, but for best results open the MAS link in Safari with the App Store app closed.

Extra methods

Installers can be downloaded and automatically repackaged into an archivable / deployable .dmg file with Greg Neagle's excellent 'installinstallmacos.py' script (requires 10.13+), available here:
- https://github.com/munki/macadmin-scripts/
- https://raw.githubusercontent.com/munki/macadmin-scripts/master/installinstallmacos.py
This script is machine-aware, and will show only the versions that the machine running the script can actually use, including that machine's fork-build, if applicable. Use macOS in a VM to bypass the hardware check.

For those currently on Catalina, you can obtain installers with the 'softwareupdate' Terminal command to download the normal 'Install [OSname].app' into your /Applications folder. Like Greg's script, this is machine-aware, so you can't download an installer on a Mac that cannot run that OS. The syntax is:
  % softwareupdate --fetch-full-installer [--full-installer-version {version}]
where {version} is 10.13.6, 10.14.5, 10.14.6, 10.15 (not 10.15.0), 10.15.1 etc. Omitting the sub-flag will get the current release.

To download an installer for 10.13+ on a Mac which cannot run 10.12 or newer, use the dowload function built into these Patcher utilities:
- http://dosdude1.com/highsierra/
- http://dosdude1.com/mojave/
- http://dosdude1.com/catalina/
Once the utility has completed the macOS installer download, decline the offer to perform the patches and quit the tool.
These all access the MAS downloads directly like 'installinstallmacos.py' but masquerades as a VM to bypass the hardware-check requirement. The installers obtained with these three utilities are checksum-perfect and match exactly those downloaded normally via MAS or other regular macadmin tools - any patching done happens *only* on the USB-stick installer they create.