As I was looking for bellows materials for my DIY Camera project (which is now on hold as I am traveling), a good option came from an unexpected place, a regular fabric store. The Ron-Loc blackout cloth, for $5.99 a yard is 99.9% lightproof and when painted black with fabric paint becomes 100%. I figured I'd share this with you, as many people seem to look for such a material. For bellows the material is on the thick side (at 0.015") but useable.

See the link in my signature for some other ideas.