I've also spent some time, off and on, looking for fabric for both bellows and dark-cloths. There are online stores selling outdoor fabrics that are worth searching, and ThorLabs sell a lightproof material that seems to be used for shielding experimental laser set-ups. You may have discovered these already, of course.
I have an Intrepid 10x8 and a darkslide due for delivery in the middle of February. Not the first on this forum, but I'll post something about the dark slide when it comes. My first reaction to their specification was that aluminium sheaths may be more subject to damage than plastic or carbon-fibre ones would be. We must assume that they know their own business.
Other goodies are promised, like a light-source that converts a camera to an enlarger, by replacing the camera back, which opens up all sorts of possibilities. No doubt we can all think of useful things that we'd like to see and perhaps we should let them know. The wider their appeal, the better chance they have of continuing to feed the LF world.
My own conclusion is that it's not quite worth it, to make dark slides up to 10x8, but after that, prices seem to soar to Ferrari and Rolex levels.