This article is from the Anime Music FAQ, by Ru Igarashi with numerous contributions by others.
It isn't clear there is much around to begin with. Also if there is something published in book form, you might be able to get it from Japanese stores, like www.amazon.co.jp.
A general sheet music site like the following might have something.
Doremi Music Publishing Co., Ltd. 3-36-4 Takada Toshima-ku Tokyo 171-0033 Japan phone: 03-5950-8220 fax : 03-5950-8246 http://www.doremi.co.jp Site is in Japanese.
Some fan transcriptions can be found at: Neko's guide to Anime Sheet Music http://home.talkcity.com/InfiniteLoop/nekosenshi/ [defunct]
Cowboy Bebop This was posted 9 Nov, 2001:
While looking for an "impish" picture of YK, I found this at James McCawley's site:
Cowboy Bebop Sheet Music
At last, sheet music for a Kanno score has been officially published. The "Cowboy Bebop Bandscore", containing Kanno's band arrangements for the tracks Tank, Rush, Space Lion, Cats on Mars, Memory, and Real Folk Blues, is available from Movic, ISBN #4896014138, retailing for 2000 yen.
It turns out there was an earlier sighting by Lynn Tse in Sept 1999:
Cowboy Bebop Band Score Movic ISBN: 4-89601-413-8 2000 yen (tax not included) released 1999.1.15
The 80 pages book is black A4 size, with Spike on the front cover and the Swordfish in the back cover. Please note that this book is a Band Score, not piano sheet music, meaning that if you want to reproduce any of the songs in the book, you will need a Jazz band. Some of the songs uses something like 15 instruments.
The songs are:
1. Tank!
2. RUSH
3. SPACE LION
4. Cats on Mars
5. Memories
6. The Real Folk Blues
Sorry about missing that, Lynn.
Note, I think this book is now out of print.
For Maaya Sakamoto and Yoko Kanno fans, this was posted on 29 Aug, 2003:
Title: Yasashiku hikeru Sakamoto Maaya Piano Solo Album, Single Collection + "Nikopachi & Hotchpotch" Publisher: kmp ISBN: 4-7732-2097-X Price: 2000 yen Description: Piano scores for all 31 tracks from Sakamoto Maaya's two single collection albums (http://www.cdjapan.co.jp/anime/topics/maaya_sakamoto/index.html). All tracks are composed by Kanno Yoko. Level: beginner Website: http://www.kmp.co.jp/Shinkan/shinkan0309.html#a
In the same posting:
Title: Piano de hiku Juunikokuki Yasou Tsuki no Shizuku ~Piano Memories~ Publisher: Yamaha Music Media ISBN: 4-6362-5817-7 Price: 1500 yen Description: Piano scores from all 11 tracks of the mini-album of the same title (http://www.cdjapan.co.jp/detailview.html?KEY=VICL-61146). The album is a collection of piano pieces (some have been rearranged for piano as well) from the "Juunikokuki" soundtrack. The score book is apparently supervised by the Ryo Kunihiko, the composer and pianist for the "Juunikokuki" soundtrack. Level: intermediate to high Website: http://www.ymm.co.jp/products2/detail.php?format=search&code=GTP25817
 
Continue to: