You can download drupal-8.0-alpha3 or clone Drupal 8 in git:
git clone --branch 8.x http://git.drupal.org/project/drupal.git
wiifm has posted a great (and short!) article on upgrading the JS Injector module to Drupal 8:
A practical example for converting a Drupal 7 module to work with Drupal 8
It covers all the important first steps you'll need to start a module upgrade. Check it out!