The only software you need is something that can speak the ACME protocol, which is what letsencrypt uses to receive your CSR (which you can generate yourself wi

The only software you need is something that can speak the ACME protocol, which is what letsencrypt uses to receive your CSR (which you can generate yourself with openssl), issue challenge tokens and then (subject to you successfully passing the challenge) give you a certificate. You don't necessarily have to do any of the above on the server for which you want the certificate. You can do it somewhere else and then transfer the resulting certificate to the intended host (either manually or with a shell script/ansible/puppet whatever).