How do I locate specific file in a post that affects ssl?

A quick squint at your site reveals that you’re serving it with Nginx, so a simple .htaccess redirect won’t work.

The following block in your Nginx config should take care of any stray locally-served files that are still coming over http:

server {
    listen 80 default_server;
    listen [::]:80 default_server;
    server_name _;
    return 301 https://$host$request_uri;
}

If you were running Apache, the following in your .htaccess before the WordPress section would achieve the same thing:

# HTTP to HTTPS
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteCond %{HTTPS} off
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [R=301,L]
</IfModule>

The above is presented in lieu of tracking down the specific file, as the page no longer seems to be throwing the mixed content error.