The Witness appliance was sometimes listed as a separate product in the download selector. You can search directly for "vSAN Witness Appliance" in the Customer Connect search bar for versions prior to 7.0 U3.
Click on the tab at the top of the product page. Search for " Witness " or " Appliance " in the search box. Download the OVA : download vsan witness appliance
The Witness Appliance version must precisely match your vSAN cluster version. Deploy the downloaded OVA via the "Deploy OVF Template" wizard in vCenter, and you will be ready to configure your vSAN Stretched Cluster. The Witness appliance was sometimes listed as a