I would first find the fabric that you like, by visiting a variety of fabric stores, and if that doesn't work, then try some custom curtain stores. Once you locate the fabric that you want to use, purchase a small sample of it, and take it to a local seamstress, and ask how much it will cost to have her sew the fabric into curtains. If your car has peculiar shaped windows, you might have to find a similar car, and take pictures of how that cars drapes were manufactured.
I know of no commercial manufacturer of drapes for professional cars, and this question has been asked many times in the past, and this is how it is usually resolved.