You're trying to block all the light? It seems like you want sheets of Duvetyne (a solid non-reflected black cloth you can get at Film Tools). I guess I don't understand exactly what you're looking for.
For Mylar, take a look at TAP Plastics (http://www.tapplastics.com). I've ordered from them a couple of times and they were good. They have mylar rolls (mirrored and clear) you can order.
Mylar is usually fairly expensive and traditionally used for its mirror-like surface. If you don