I agree, to never cheat on any archival materials. Try your local art supply store; see if they can cut whole (32 x 40 inch) sheets to size. If not, shop around some sources online; there are many to choose from. I've ordered from Light Impressions for over twenty years, with great satisfaction. They have a good selection, and are rarely backordered:

http://www.lightimpressionsdirect.com/

Good luck.