For v13.x - The patch is located here: https://ftp-public.imperva.com/SecureSphere_Patches/v13.2/patch20/SecureSphereV13.2.0-x86_64-Patch20_0.x.enc
For previous versions - 
For instructions on upgrading SecureSphere MX and Gateway on Amazon, please follow the procedure located in the relevant AWS Configuration Guide:
WAF-AWS-BYOL-Configuration-Guide -  https://ftp-us.imperva.com/Downloads/SecureSphere_Documentation/v13.2/
WAF-AWS-On-Demand-Configuration-Guide - https://ftp-public.imperva.com/SecureSphere_Documentation/v13.2/
Database-Security-AWS-BYOL-Configuration-Guide - https://ftp-us.imperva.com/Downloads/SecureSphere_Documentation/v13.2/