The Internet Guide to Cels is my primary 
resource, I see that other collectors have kindly 
posted the link.  I also bid on EBay and Yahoo to 
get them.

One note of caution though:
I was recently burned buying what I thought was a 
production cel of Belldandy from the anime series 
OH! My Goddess.  Turns out that it was actually a 
reproduction cel, which is worth far less.  I 
would strongly suggest only dealing with 
reputable dealers, and if you are using PayPal, 
to only send money to VERIFIED people.

- Jae