I am looking to buy one of the chalk-boards used in filming hollywood
movies.  Typically somebody holds it in front of the camera and snaps
the top part onto the bottom part around the time the director yells,
"action!"

minermbel...

Thanks for your question.  The item you are referring to is known as a
clapboard, clapper or slate.  It has many uses in film making.  It
identifies the shot by listing the film title, the names of the
director and director of photography, the scene and the take number. 
The top "clap" portion is used in order to synchronize audio and
visual.

You can purchase clapboards at any number of places:

Clapboard Slate
http://www.writersstore.com/product.php?products_id=957&cPath=24_57

Great Big Movie Clapboard
http://www.greatbigstuff.com/clapboard.htm
Oversized (28 x 24) clapboard

Clapboard Gifts
http://www.take1.com/clapgft.htm

Thanks again for your question and I hope the above information has
been helpful.  If you need any additional clarification, please let me
know prior to rating my answer.

Regards,

-THV

Search Strategy:
movie clapboard
clapboard

References:
Internet Movie Database-Movie Terminology Glossary
http://www.imdb.com/Glossary/C

Comprehensive better than I could have hoped for!

Thank you very much for the 5-star rating and the generous tip.  I am
glad I was able to be of assistance.

Lights! Camera! Action!

-THV