it's possible to obtain a free SSL certificate to activate HTTPS on a website? I mean, not an auto-certificate, but a certificate verified in order to have in the browser the green lock when a user is connected. Thanks
Check cloudflare.com, if you add your domain dns to cloudflare they will give you free ssl certificate without any payment. Beside this they also add other protections on your site for free from DOS and other attacks.
Click here to check how to setup domain on cloudflare
Beside This you can also check https://letsencrypt.org/. It's also pretty good. You can self sign the certificate for free and use it in your website. But as per my experience sometime it doesn't perfectly work with some browsers. In my case it was Mozilla Firefox. Plesk, Cpanel and Vesta Cp Server Control panel support an easy to setup extension for this. Which you can easily find if you are using it any of this panel, if not then you can still use it by direct installation.