In linux you can just use the openssl commandline tools to download any certificate from a remote web-server and store it in a local file.
Show the certificate chain and public key of a web-server
openssl s_client -connect slash4.de:443 -showcerts
To directly extract the certificate and save it to a file you can use these commands (all in one line):
openssl s_client -connect slash4.de:443 -showcerts </dev/null 2>/dev/null | openssl x509 -outform PEM > slash4.pem