SSL certificate issue

Our code throws and exception when trying to call Smartsheet API:  

SSL Exception: error:1416F086:SSL routines:tls_process_server_certificate:certificate verify failed

The Rest Client has a logic to verify certificate using the windows system certificate store . It looks to me the CA certificate for Smarthsheet API is not available in the truststore.

My question is: where can I download the root/intermediate CA certificate for Smartsheet API so that I can import them into the truststore?

Or am I looking at wrong direction?