Got SSL, now all requested files are blocked (mixed content)

I have clients who use HTML on their pages, that I provide. That HTML links to files on my server (JS, CSS, images, etc).

Example of what I give them:
<link type="text/css" rel="stylesheet" href="http://www.example.org/this.css" />

I just got an SSL, so my site is now https. However the HTML on their server, that I gave them, is still http when requesting files from my server.

Because of this, they are getting mixed content warnings and the content is blocked. Like this:

Mixed Content: The page at 'https://www.example.org/' was loaded over HTTPS, but requested an insecure stylesheet 'http://www.example.org/file.css'. This request has been blocked; the content must be served over HTTPS.

I can't have all of my clients change all of their links on all of their pages to "https" so that warning/blockage is prevented. That would be a nightmare.

My host is GoDaddy. My server is a Windows server, IIS: 7.0, ASP.Net Runtime Version: 4.0/4.5.

How can I resolve this on my end through web.config? My current rules are:

<configuration>
<system.webServer>
    <rewrite>
        <rules>
            <rule name="HTTP to HTTPS redirect" stopProcessing="true">
                <match url="(.*)" />
                <conditions>
                    <add input="{HTTPS}" pattern="off" ignoreCase="true" />
                </conditions>
                <action type="Redirect" redirectType="Permanent" url="https://{HTTP_HOST}/{R:1}" />
            </rule>
        </rules>
    </rewrite>
</system.webServer>

What I want to happen is have all outside http requests to my https server, to be allowed.

Thanks!