Finding Comodo root and intermediate certificates can be a challenge. If you start with Google, you might find a promising knowledge base article with a direct link, but then be unable to find the certificates behind your certificate.
It seems the trick is to start with this “master page” of Root & Intermediate(s):
Unless your cert is very old (unlikely), check out the left-hand, SHA-2 column:
Click on each certificate and try to match the serial number to the certificate in your chain. Here’s the serial number as shown in Firefox: