Where can I download the /usr/dict/ packages?

You can look at GNU's aspell dictionary list

Or simply install the sources for the packages, which will give you what these have been made from.

There are several free dictionary databases, if I gather correctly that this is what you're after.

The ones Debian GNU/Linux provides (in /usr/share/opendict/dictionaries) are from FreeDict (also see here, and here for tarballs).

Mike Chirico compiled an enormous English word list. Having such an enormous list can make a difference in some applications.