How do I load specific page template for /store-locator
There are many possible ways to solve this problem.  My suggestion, however, would be to use the Page Manager Existing Pages module.  This module will let you easily override the callback at that URL and place the content for the page in any region of your Panels layout.

If you want to do it the same way as you've been doing it for other pages, then there will already be template suggestions automatically provided for you, derived from the path.  So if the path to the item is sitename/store-locator then the template suggestions that will already be automatically created for you are page__sitename and page__sitename__store_locator.  You need only to create those tpls.

That said, I'd still generally recommend the PM Existing Pages module for this type of functionality.