The studio's who produce the cartoon have all 
rights to the actual cel's.  They will usually 
hold onto them until after the production of the 
cartoon has ended, just in case they want to use 
certain one's for bank cels and such.  Once the 
production is over the studio's will sometimes 
sell the cel's to get more intrest in the videos 
and movies that may be realesed, say a collection 
of a certain portion of the series.  They send 
these to a private dealer's or sell them 
directly.  After the private dealer, they usually 
get sold wholesale, and that is where most us 
dealers get their cel's unless they have good 
contacts in Japan.

if i am wrong someone please correct me.