I used this tutorial to integrate bootstrap in my project:
https://laravel-news.com/2015/10/setup-bootstrap-sass-with-laravel-elixir/
This places an app.css
In Laravel 5.4 what I did is
mix.copy('node_modules/bootstrap-sass/assets/fonts/bootstrap/', 'public/fonts/bootstrap');
And in resources/assets/sass/_variables.scss or app.scss
resources/assets/sass/_variables.scss
app.scss
$icon-font-path: '/fonts/bootstrap/';