Fiberglass G10 is a great material for testing out a design if you don't want the expense of having it cut from carbon fiber. It is also no conductive, so it works well in spots where you would be concerned that power could be conducted though the part.
We currently have stock on the following thickness:
0.8mm, 1.2mm, 1.6mm, 3.2mm, 2.6mm, 6.3mm
The material comes with a black finish on both sides.