He means matting board, like framing material. I use the same stuff. Works great. You can find it at Micheal's or Ben Frankln's, or any framing/craft store. It comes in 2 variaties: Normal ($7.99 for 3x5 sheet) and Acid-Free (13.99 for 3x5 sheet). To get them by the sheet, just ask for however many sheets you need at the framing desk and they'll get it for you. Again, it is good stuff. very sturdy and yet easy to cut with an X-acto knife.