The files that get_file references are passed to Heat as part of the request to create the stack, so local files work fine. If you're using python-heatclient, you can just use a local path to the file and it will automatically translate it to an absolute URL and pass the contents as part of the request.
If you're using a different client, you'll need to ensure that it passes the files' contents in the files section of the request, which is a JSON object where the keys are the names of the files that are referenced by get_file.