Only one I know of that is for sale right now is 
at 
http://www.kcat.zaq.ne.jp/collection/Cowboy.htm. 
For any Julia cel, it will run you a minimum of 
$500, and upwards of $1000. If you want to make 
offers on cels, you can go to 
http://www.celstation.com/. That site has an 
updated listing of all the sites that carry 
Cowboy Bebop cels. Good luck!