Chi is correct about the digital coloring.  Walt 
Disney studios still in most cases uses cels for 
their television series, but their last feature 
film to use cels was "The Little Mermaid".  
The "cels" you are speaking of from newer feature 
films are publicity cels, limited editions, and 
sericels, and etc., produced soley to be marketed 
and sold as collectable items.  

As for "Treasure Planet", yes, production 
drawings and sketches are available and I know of 
a private collector I do a lot of business with 
that has some and access to more.  I'd have to, 
however, ask him if he wouldn't mind me giving 
out his email address if you're interested in 
truly buying some.