Sometimes Mandarake has sketch sets for auction 
(http://ekizo.mandarake.co.jp/auc_e/index.do), 
although I do not see any Naruto sketches there 
at the moment.  Animegame.com is a cel dealer 
that appears to have some Naruto stuff right 
now.  A good place to start for a list of cel 
dealers might be http://www.cels.org, but you 
might want to ask on http://www.anime-beta.net 
which is a pretty active cel collector fourm.  
Hope this helps :)