I carried cels for a few years and bought them from the studios of the artists who originally drew them. One of the most popular artists, of course, was Chuck Jones, who did Bugs and his gang, Tweety, Road Runner & Wiley Coyote, and many others.
His daughter handles the organization and can be contacted at:
Linda Jones Enterprises
POB 2319
Costa Mesa, CA 92628-2319
I haven't dealt in animation cels for a few years so I don't know if they have a website or not.
Turner Home Entertainment also did some LE's of animation cels by many of the cartoonist illustrators like Maarcia Ferlig who worked on Alice in Wonderland, Peter Pan, Dumbo, and many others. Myron Waldman was another illustrator that was marketed by Turner in the early 90's and he was noted for his work with Popeye, Casper the Friendly Ghost, and the old Superman cartoons to name a few.
Good luck in your search. I am sure that there will be others that will add to the information for you.
Framerguy