Where can I find the source_sync.sh script?
You should find this script on host in Linux_for_Tegra diretcory of your JetPack installation.
If installed through sdk Manager
It was in /home/accountname/nvidia/nvidia_sdk/JetPack_._/Linux_for_Tegra. Right?
That is correct.
please also specify the release tag while you’re using the script to get the sources,
you may see a list of the available release tags, for example,
$ git tag –l tegra-l4t*