Take a look here, they have some nice Slayers Cels...

http://www.nichibeianime.com/cels/slayers/index.html
http://anime-link.com/slayers/
http://www.animegame.com/slayers.htm
http://www.storage.asylum-anime.com/cels/slayers/
http://www.animeed.com/slayers.htm
http://www.animemuseum.biz/shop/index_plist.php?lan=en&show=all&title=0763&p=1

Of course, you can try to bid on Yahoo Japan throw
a deputy service (Animechaos, rinkya, ...):
http://list5.auctions.yahoo.co.jp/jp/2084034858-category-leaf.html?at=true&alocale=0jp