[Answer]-Select popup or modal for huge select lists

The Django admin can do this already, wherever you have a ForeignKey field in the ModelForm the admin site will show the + button and popup selector functionality you want.

Here is an article about how to achieve the same thing in your own views outside the admin:
http://sontek.net/blog/detail/implementing-djangos-admin-interface-pop-ups

…author has released the code as a reusable Django app:
https://github.com/sontek/django-tekextensions

Here is another app which does more but also includes the functionality you’re looking for:
https://django-autocomplete-light.readthedocs.org/en/latest/addanother.html