I buy mine locally from Dickblick at times.... 
you can visit their site at www.dickblick.com.
One of my favorite online stores is Utrecht, as 
they have excellent customer service and some 
awesome deals at times. Their URL is 
http://www.utrechtart.com/dsp_view_search.cfm

You might also wanna try www.misterart.com 

http://www.cartooncolor.com/ sells studio cels
but personally, I still recommend the Duralar 
brand...
Also, if you are into airbrushing or other art 
supplies, check into 
http://www.dixieart.com/FineArts/FineArtMain.html
There is a airbrushing community that speak very 
highly of this site and they provide excellent 
customer service as well.

Hope that helps