I wandered onto www.darkmount.com one day and saw-
 GASP!- a Starscream cel!!!! (No, I wasn't able 
to get it...trade only...:P- Still there!)
http://www.darkmount.com/stuffforsale
I did engage in a email conversation, though, and 
Tursi told me that the best place to look for G1 
cels is Botcons and things like that. She also 
said that she was told most of the cels were 
destroyed by the company- cleaned and 
reused..*sobs*...
However, some do still exist....so Ravage may 
still be out there somewhere..:)
So far, ebay has been my best bet- that's where I 
got the Swindle pre-production cel that's on my 
page now.
Good luck!