If you have access to OpenSSL, try

openssl s_client -connect {HOSTNAME}:{PORT} -showcerts

replacing {HOSTNAME} and {PORT} with whatever your values are.

In order to download the certificate, you need to use the client built into openssl like so:

</dev/null openssl s_client -connect $HOST:$PORTNUMBER -servername $SERVERNAME \
    | openssl x509 > /tmp/$SERVERNAME.cert

That will save the certificate to /tmp/$SERVERNAME.cert.

The -servername is used to select the correct certificate when multiple are presented, in the case of SNI.

You can use -showcerts if you want to download all the certificates in the chain. But if you just want to download the server certificate, there is no need to specify -showcerts. The x509 at the end will strip out the intermediate certs, you will need to use sed -n '/-----BEGIN/,/-----END/p' instead of the x509 at the end.

</dev/null indicates that nothing should be sent to the server, so that the connection is released.

openssl x509 removes information about the certificate chain and connection details. This is the preferred format to import the certificate into other keystores.