Princess Tutu was a CGI series, so there are no cels, only sketches, and these are very, very scarce and avidly fought over. There are also original watercolor backgrounds, but I have not seen any of these for several years, and I'd expect these too to meet with very competitive bidding.
Can't speak for Kaleido Star but I believe that too came out just after studios moved to the CGI format. Art from this series also seems very scarce.
The Snake's Den has some, all from one episode (which usually indicates that someone slipped a set out of the studio's back door).
