We just finished a one-room school exhibit for the Iowa sesquicentennial that
includes a chalkboard.  We went through a process of looking for a slate board
as well.  There are a few things you need to consider:

        They are heavy!!!! very heavy, make sure the wall you have will support
it.

        I wouldn't want to ship from maine to oregon :-)

        The pool table idea is good, but sometimes they cheat and more than one
piece of slate is used, but that's ok, because chalk boards could also be made from
more than one piece of slate.

        we ended up useing a contemopry chalkboard, but removed its modern
framework and replaced it with a wooden one.  of course you would want to study
some photos and reproduce something appropriate for you area and era.

being part of a university made finding a chalkboard easy.  this may be a
resource for you as well.  this university had several chalkboards left over
from remolding jobs around campus (who knows, one of the left overs may
actually be slate).  i would suspect any university or college in your area
has some left overs, and for a good casue such as yours, their generosity should be
forthcoming.

good luck

Terry Vidal
University Museum
University of Northern Iowa