affordable mat board

I need some 18x18 sheets of mat board. I checked at Wal-mart, kmart, and a local scrapbooking store, but nada. A gallery agreed to sell me a piece for $12, not an option.
I am only using these to place on top of an 18x18 piece of plywood which I will use to cut foam board or Gatorfoam for labels and panels in a small museum. Of course, in time the mat board will have to be replaced so I need a good price.