If you want the entire skull gas cap, you'll have to find a way to powdercoat it. As the peeps above stated, there are a few medallions on ebay that have been powdercoated black. If it doesn't need to be the skull gas cap and it could be just any black gas cap, Pro-One makes a variety of inserts for their black gas caps. I have a low-pro black gas cap that I got from Pro-One and it is really nice quality. RSD also makes a black gas cap.
I sanded my willie g skull cap and rattle canned it. Looks sweet. I'll get a pic and put it in my album soon. I did take the cap to a powder coater and was told it couldnt be done because of the threads. It proved smarter then the both of us cuz neither of us could figure how to take the threaded part off. I'm happy with my cap