The studio did release the Urotsukidoji cels many 
years ago.  Since it was such a hentai classic 
and extremely famous back then in Japan, many 
Japanese die-hard fans have bought them into 
their collection.  Only a small amount of 
Urotsukidoji cels have gotten into the hands of 
US and Europe cel dealers, they have sold out 
ages ago.  And nowaday, expect to pay very high 
prices if you can find them.