It's a standard 7" light if I am not mistaken. You should be able to find what you are looking for on Ebay or maybe Alibaba or something. But there isn't anything specifically made for the GZ.
You may also want to look in to the perforated vinyl covers. They will look solid green from straight ahead but will still allow all the brightness of your headlight to shine through.
http://www.alloutgraphics.com/products/ ... overs.html