Well, everyone seems to have suggested the best 
places. Anime Chaos and ANimanga would be good 
places to starts, and Asylum Anime just updated. 
Also try Japanese webpages, as they usually sell 
cels cheaper. Then, there is always YJ (Yahoo 
Japan) that also sells cheaper (you would need to 
use a deputy service or third party, such as 
Celga). Hope this helps!

-Misha-chan