The intermediate certificates you need are available at two places.
You can find and download the intermediate certificates on the same page in the Customer Portal where you downloaded your end-entity certificate:
At the bottom of the screen are download links of intermediate certificates you need, including two bundles: ALL-CERTIFICATES.ZIP and CERTIFICATE AUTHORITY BUNDLE.
The ALL-CERTIFICATES.ZIP file includes the following certificates (your server certificate and three CA certificates):
The CERTIFICATE AUTHORITY BUNDLE contains two CA certificates in a single .crt file. It is designed to be used for Apache openssl servers. You can download this file and use it directly in the Apache openssl server configuration line SSLCertificateChainFile:
Note: Make a backup copy of your SSL certificates and keep them in another location.