Where can I buy or download dictionary files for a spell checker? We
are building a spell checker and need the full English dictionary (in
British and American dialects) also legal terms, medicinal, slang, and
"culture" (places, people, other proper nouns that may not be in the
regular dictionary. Thank you. They must be complete files and easily
used with spell checker software such as possibly ASPELL. Also if you
have any better ideas on how to build a spell checker please let me
know. We need one on par with Google's. Thanks!

Foxspell Checker (http://www.hallogram.com/foxspell/) includes a
dictionary of over 195,000 words.  I have been using thie program for
over 5 years.

Moby words is a public domain listing of 610,000+ words and phrases. 
It claims to be the largest word list in the world.
http://www.dcs.shef.ac.uk/research/ilash/Moby/

The ReallyBigWordlist contains nearly 3 million distinct entries.
http://www.morgud.com/reviews/miscellaneous/ReallyBigWordlist-ANSI.asp