I order clay from Sheffield Pottery in Sheffield, MA (pardon my spelling).
They have a website and the price is excellent (the more you buy, the less it
is) as is the quality! Finally this year they won our clay bid so all of our
clay will come from there. In the past I had ordered it seperately because the
clay was less expensive, better quality and I could run up and pick it up
myself (I am about 2 1/2 hours from there). A fun trip to their show room as
well as other stops along the way.
I usually order one box (50 lbs) per each class, but I don't use that much.
For most projects I find 25 lbs per class will just about do it. I might use
a little more for larger sculptures, for small projects like pinch pots, I use
about 1/2 of a 25 lb block (for lack of a better word) per class. (my
classes may have up to 22-23 kindergarteners). I make sure I always have l clay
left over so I can start the year without worrying about scrounging up clay a the
beginning of the year.