Can't find wp-blog-header.php?

I’m trying to create a new file to use with an ajax request. In my templates folder, I added a file named sale-prices.php that needs to make a WP_Query. In a previous theme I created without Roots/Sage, I was able to successfully add:

 require('../../../wp-blog-header.php');

at the top of the page to make the query work. I can’t seem to get this to work in Sage, no matter how I try and edit the …/ to find the root directory. I get this error,

Warning: require(../../../wp-blog-header.php): failed to open stream: No such file or directory

Is there a different method I should be doing to get this to work?

Check out http://ottopress.com/2010/dont-include-wp-load-please/ and https://codex.wordpress.org/AJAX_in_Plugins

Thanks Chris! I’ll look through those links.