they're so rare... I would almost suggest getting 
a "good one" and then if you find a better one 
selling what you had. but then you might have 
problems selling since not alot of people collect 
that series... tough call.
I've hardly EVER seen watership down cels.
sorry havent heard of plague dogs cels... I saw 
it once though :)

mmm... that's tough though, good luck!!