There are several online stores mainly from Japan 
that sell them, but I haven't found any cel that 
have really caught my eye.  I have gotten one of 
the ending cels from Anime Taro, though.  And 
Anime Cels World has had some nice ones in the 
past.  If you're looking for cheap, I suggest 
checking E-bay (not the best cels, but very low 
cost).

And a suggestion for hunting, if you don't see 
Detective Conan, look for Meitantei Conan.

Laters,
Sumeragi