DIY Custom Camera Bellows Kit
This is cool!
Though not my way of bellows making, but it is good for starter.
Camera Bellows Outer Cover Fabric
It is too thick to me, but should be very good material for size larger than 4x5.....
(11/04 add) there is going to provide a much thinner material, when Sorin solved the lightproof problem and cost problem. In my opinion, as long as you let buyers aware that 3 layers of bellows construction is 100% lightproof is enough, no need for additional treatments, that will increase thickness.
Fine Grain DIY Ground Glass Kit
and this one shows you how to grind your own ground glass, amazing!