So there are no aftermarket fenders specifically for the DS. As stated in the unofficial manual the Honda XR650 shares the same bolt pattern, however they're generally only available in red, white, or black, and they look a bit dated.
I saw a carbon KTM SX125/250/450 fender on Ebay and decided to take a gamble on it, being prepared to drill new holes if need be. Turns out no drilling is required, I don't think the pattern is exactly the same but it uses slightly larger bolts so there's some wiggle room. There were a few modifications required, the posts for mounting the KTM's front plate needed to be cut off and to clear the DS's headlight I needed to add a spacer to bring the fender down slightly. The material I made it from was 10mm thick which was just enough. I was also expecting to have to cut down the rear but it clears the plastic on the battery with probably around 10mm at least to spare.
As for the whys, it's quite a bit stiffer, wider, it looks cool, and stickers will stick to it. I tried to put an adhesive L plate on the stock fender so I can get rid of the one sticking out the side but the finish on it just wouldn't have it.