There's not much "free SSL providers" out on the market. The most popular ones is LetsEncrypt and Cloudflare (which is free). I recommend using them if you don't want to pay for a certificate.
But if you want to look professional and if you don't trust free services, I recommend using (globalsign.com) and buy a SSL Certificate there.
As always, there's big difference between free SSL Certificates and paid ones.
I hope I answered your question!