I have a Django app running on Heroku/Cedar, configured as per the instructions at https://devcenter.heroku.com/articles/django
Using gunicorn as per Heroku\'s instr
If you use runserver and configure your app with DEBUG=True, then it will serve the admin files just like on your development machine. However, this is definitely not the recommended way to do it, and I would suggest that you put them on S3.
Using the django-storages app it's very easy to configure collectstatic to automatically push all the admin files to S3. You can find directions here