In addition to over 60 cut sizes, Frame Destination now sells mounting board by the sheet. On our website here:
http://www.framedestination.com/Mount_Board.html

Just select "32x40 Raw Sheet" size. Since we are not doing any cutting on the foamcore or rag board you can save a lot of money. You can save even more if you purchase 5 sheets or a full box of board at 25 sheets. We carry:

Acid-Free Foamcore Mount Board
Non-archival Foamcore Mount Board
3/16″ or 1/8″ Thickness
Bainbridge 2-Ply Artcare™ AlphaRag Mounting Board
Economical 1/16″ Papermat
Black Acid-Free 3/16″ available

Cheers,
Mark