For the http://www.diybookscanner.eu or http://www.diybookscanner.org kit, you will ideally need two 280x356mm sheets of anti-reflective low iron 3mm glass.

Hints

• Most picture framing shops will be able to get you 2mm, but that is really too thin.
• Good anti-reflective glass lets over 98% of the incoming light get through. Any other glass gets to about 92%, the rest being reflected.
• If the shop is offering you cheap "anti-reflective" glass, it is most probably just anti-glare. Anti-glare glass scatters the reflection, making the result of your scan look fuzzy. This gives you worse results than just plain float glass, so buyer beware!
• Look at the glass sideways. If the reflection is dark green, you probably have normal float glass in your hands. If the reflection is blue, purplish or brown, you may have be holding a more interesting kind of glass.
• There is anti-reflective glass with a sputtered coating on one side, or with a "bath" coating on both sides. These double sided coatings are a lot more scratch resistant.