When I'm not taking a commission, I ALWAYS go to 
screenshot galleries...or pull out a cd of 
digitally encoded anime and go through it frame 
by frame, but that sucks if you don't know 
exactly which episode the scene you want is in.  
There's a much bigger diversity, and you're more 
likely to find a picture from that scene you 
ADORE. 

For example, with Utena, I've seen three or four 
good cels from the movie and a few more from the 
tv show, but right now I'm working on a huge cel 
from the last episode where Utena is reaching for 
Anthy in the coffin.  I've never seen cels that 
good in a gallery.  

Besides, since I'm an artist in my own right, I 
can improvise and reproduce a cel even from an 
itty-bitty 1" wide screenshot (which is hard, but 
possible if its a portrait)--if you don't believe 
me, the two Black Lady portrait cels on my 
website were done just that way, and the full 
body one was done from a .5" wide douga in a SM 
art (not cel) gallery.  Of course, the Rei cel I 
posted a while back HAD to be a screenshot, 
because there's no cel to trace it from ^_^.

So don't knock the possibility of making cels 
from screenshots =P