Within my themes page.tpl.php file it has a reference to a lot of variables like $head and $local_styles. What file are they stored in?
The values are not stored at all. Instead they are generated dynamically at run time and feed into the template. The list of variables is just there so you will know which variables exactly are available in any particular template.

If you want to examine their content, you can add

drupal_set_message(print_r($variable, TRUE));


to the template, and simply load the page. If you install the Devel module, you can get much easier to read output like this:

dpm($variable);


I frequently use dpm in my everyday work when debugging or building new code.