I'm sure some of them are on a booru, and I've seen some (very few) uploaded to artists sites.
The sauce to these images is yahoo auction japan. For a year now I've combed every "Rozen Maiden (hand-drawn) illustration that's been sold, saving images from nearly every auction.
Some of these pictures were sold for over the equivalent of 300 USD, no joke.
Since people rely on this to make (some) money, you can guess that people put a lot of effort into making these images good, therefore sellable.
The only problem is that most images have a watermark with the artists name or the word sample, partially to prevent assholes like me from collecting/full enjoying the pictures and partially to prevent the equivalent of the image from being printed out for free.
Regardless, this can be overlooked, especially since the highest priced ones have no watermark. It's not necessary when people are willing to pay out the ass for an absolutely magnificent original.