To get the older version of any app and add-on which is not available on https://splunkbase.splunk.com/ you need to contact the account owner or sales engineer. They will provide the correct/compatible and supported package for the same.
No We are using the customized version of Symantec Endpoint Protection Version 2.2.0 so need to download the original add-on version from splunk base. So do you know from where we can download this version.
You can contact to your Splunk sales rep to request the packages for the previous version. Also in new version they provide release history which may helpful to you. Refer: