Cloudflare: Offers a free basic service, with the possibility of having a free certificate for your web site. To use this service you need to be able to change the nameservers of your website.
Https for free: This tool, available on dotmaui.com, offers the possibility of having a free certificate valid for three months in a linux environment. Once expired it will be enough to regenerate a new one, always free of charge. To use this service you need to have access to the server via SSH.
CertBot: Certbot is an easy-to-use automatic client that fetches and deploys SSL / TLS certificates for your webserver. To use this service you need to have access to the server via SSH.
If there is not a valid solution for you among these three choices, consult the list of software and services that use Let’s Encrypt to get free SSL certificates.