When I updated to the latest build of slicer from the website build 9/8/19. I cannot find the SlicerIGT in the extensions library.
We are currently having some instability with the factory build system, so the latest releases may not have all of the extensions. We will have this fixed hopefully in the next few days. I recommend a slightly older nightly:
http://slicer.cdash.org/index.php?project=SlicerPreview&date=2019-09-05
Under Nightly-packages you will see entries for the platforms, and a yellow package icon that will take you to a download link for the installer.
On the CDash instance, you can filter for the extension name for a particular data to confirm that it is available: http://slicer.cdash.org/index.php?project=SlicerPreview&date=2019-09-05&filtercount=1&showfilters=1&field1=buildname&compare1=63&value1=SlicerIGT
For details on the factory status, see: Update on Build factory status