I found a new-old-stock kit spoiler on eBay. It is VW-branded but I think it is made by Votex. The part number is: ZVW353003
In the box is a bag of nuts and bolts and also a template for where to drill the three holes needed to mount it.
It comes pre-painted, but at this point they are probably so rare that you will need to buy what you can find and paint it later.