Question

Where to find the root and intermediate certificates for Comodo (Sectigo) certificates?

Answer

SSL installation requires root and intermediate certificates forming a "chain of trust".
If at least one component is missed chain will be marked as non-trusted by some browsers.

Please, find the updated list of all the CA intermediate and root here.

"Comodo" renamed into "Sectigo" since 01-11-18.
Since 14-01-19 major part of Comodo products changed root certificates.
Products active after 14-01-19 use new root during activation and reissue. 
Products active before 14-01-18 using old "Comodo" root for activation and use the same for reissue.

Actual for certificates issued before 14.01.19 and actual for their further reissues.

EssentialSSL (SHA-1 / SHA256)

EssentialSSL Wildcard (SHA-1 / SHA-256)

PositiveSSL

PositiveSSL Wildcard

PositiveSSL Multi-Domain

EV SSL (SHA-1 / SHA-256)

EVSSL multi-domain

InstantSSL

InstantSSL Pro

PremiumSSL

PremiumSSL Wildcard

Unified Communications Certificate (UCC)

Actual for certificates issued since 14.01.19 and actual for their further reissues.

ROOT Root: Sectigo InstantSSL (issued since 14.01.2019)

INTERMEDIATE Intermediate: Sectigo InstantSSL (issued since 14.01.2019)

ROOT Root: Sectigo InstantSSL Pro (issued since 14.01.2019)

INTERMEDIATE Intermediate: InstantSSL Pro (issued since 14.01.2019)

ROOT Root: Sectigo PositiveSSL (issued since 14.01.2019)

INTERMEDIATE Intermediate: Sectigo PositiveSSL (issued since 14.01.2019)

ROOT Root: Sectigo PositiveSSL Wildcard (issued since 14.01.2019)

INTERMEDIATE Intermediate: Sectigo PositiveSSL Wildcard (issued since 14.01.2019)

ROOT Root: Sectigo PositiveSSL MultiDomain (issued since 14.01.2019)

INTERMEDIATE Intermediate: Sectigo PositiveSSL MultiDomain (issued since 14.01.2019)

ROOT Root: Sectigo EssentialSSL (issued since 14.01.2019)

INTERMEDIATE Intermediate: Sectigo EssentialSSL (issued since 14.01.2019)

ROOT Root: Sectigo PremiumSSL (issued since 14.01.2019)

INTERMEDIATE Intermediate: Sectigo PremiumSSL (issued since 14.01.2019)

ROOT Root: Sectigo PremiumSSL Wildcard (issued since 14.01.2019)

INTERMEDIATE Intermediate: Sectigo PremiumSSL Wildcard (issued since 14.01.2019)

ROOT Root: Sectigo UCC EV (issued since 14.01.2019)

INTERMEDIATE Intermediate: UCC EV (issued since 14.01.2019)

ROOT Root: Sectigo EVSSL (issued since 14.01.2019)

INTERMEDIATE Intermediate: EVSSL (issued since 14.01.2019)

ROOT Root: Sectigo EVSSL Multi-Domain (issued since 14.01.2019)

INTERMEDIATE Intermediate: EVSSL Multi-Domain (issued since 14.01.2019)