Perhaps there is a recycle/resourse place in your area.
We have one at the Children's Museum in Kansas City.
Printers, Frame shops, Envelope Companies and Hall Mark
Cards donate materials that we can purchase at a very
low price. I often find paper to use as watercolor or
printmaking paper there. If not, perhaps you could contact
the larger firms and ask about donations of extra stock.
Often their idea of extra stock comes 44 x 22 inches,
4 foot deep on a wooden skid.
Worth the try, Woody in KC