Manually enabling SSI on your server
If you're on a Linux+Apache server that probably supports SSI but just doesn't have it enabled, you can try manually turning it on using the magical little file called .htaccess. Simply create an empty txt file called .htaccess, and add the below to it:
AddType text/html .shtml AddHandler server-parsed .shtml Options Indexes FollowSymLinks Includes
Then, upload this file to the root HTML directory of your server account, enabling SSI across the entire site for any web page with a .shtml extension. To isolate SSI ability to just a specific sub directory, upload this file to that directory instead.
Now, assuming this works, you should now make sure your web host allows you to manually turn on SSI. Some hosts may not like the idea of you turning on a feature without paying more, so to be safe, you should check with them first.
Enabling SSI on regular html pages
Now, the above requires that your webpages be named with a .shtml extension in order for SSI to be enabled for that page. If you're going to be using SSI across the board, you may want to consider just turning on SSI for regular .html and .htm pages, so you don't have to rename your files or follow the .shtml convention. To do this, add the below code to your .htaccess file:
AddHandler server-parsed .html AddHandler server-parsed .htm
Viola, SSI should now be enabled for regular HTML pages as well!