Do you have a lot of sub-domains or need to cover your new sub domain without getting a new SSL? You can generate a wildcard SSL from our Let's Encrypt SSL generator for free.
Do you only have a single domain where you site is hosted on? You can get a free from GoGetSSL in our client area and the best part? It is even trusted on old browsers like internet explorer.
Do you want your sub domain to be automatically secured on a globally cached CDN? You should consider using Cloudflare. More details on their website.
Note: Cloudflare doesn't support sub-sub domain (eg:sub.sub.example.com), you may need to unproxy the connection to server and use one of our ssl.