All you have to do search around on these pages:

http://www.cels.org/

or check on Yahoo JP and eBay.

There are many cels on dealers sites and even on 
collectors pages that have for sale page, it 
takes time to go and look around on all the pages.
What do you mean with "good cels"? Which 
characters? If you define your "good cels" and of 
which characters it would be much easier to give 
you certian websites. Also how much are you 
willing to spend? If you can give more info on 
all that it's much easier to help you.

Kata