Hi,
Thank you for posting in Symantec community.
I would be glad to answer your query.
On the Symantec site you can see only the latest version of SEP. Legacy versions are not available however support can provide you.
Support team will upload the legacy version on FTP server and will provide the link to download.
Easies way can be, log a webcase and ask them to provide SEP RU5 version.
How to create a new case in MySupport
http://www.symantec.com/docs/TECH58873
OR
log a case with support.
Contact Symantec Customer Care on
http://www.symantec.com/support/assistance_care.jsp
Symantec always recommend to use the latest version of SEP.