What you are probably looking for is muscovite mica.  This is a mineral
that splits into very thin, transparent sheets, which was used extensively
for windows in furnaces, ovens, etc.  Where you can get it depends on how
big a piece you need.  Smaller sheets can be purchased from mineral
dealers, or from electronics suppliers, as it is still used for high
voltage insulation.  If you can't find the right piece, give me a yell at
[log in to unmask], or by phone at (904) 471-4578.

Robert Krampf
Science Education Company
[log in to unmask]
http://members.aol.com/krampf/home.html