Does Linode offer SSL Certificates? Can I use a third-party SSL?
Can I purchase an SSL certificate directly from Linode? If not, can I buy an SSL from another Certificate Authority and use it on my Linode?
You don't have to purchase anything, use Let's Encrypt.
While Linode is not a Certificate Authority and doesn't offer SSLs for purchasing, you can use any 3rd party SSL certificate on a Linode server.
You can install an SSL on many different Linux configurations. Here are a few guides for the most popular distributions and web servers:
CentOS: SSL Certificates with Apache on CentOS 7
Ubuntu: SSL Certificates with Apache on Debian & Ubuntu
Each web server (Apache, Nginx) will need to be configured to work with your third-party certificates, depending on your websites and requirements.
One of the easiest methods for obtaining and installing an SSL certificate is through Let's Encrypt. The Electronic Frontier Foundation (EFF) has developed Certbot, a free and automated tool for creating Let's Encrypt SSLs and configuring them for different web server software. Certbot runs on your Linode and can be downloaded on Certbot's website.