I seem to recall a certain webpage that had a lot 
of fantastic Last Unicorn cels saying that the 
person used to go to the studio a lot while it 
was being made and bought a couple of cels 
there.  So I suppose it depends on the studio and 
if you have any friends there... but these days 
it  probably has more to do with how big cels 
have gotten as collectables since the time of the 
unicorn.  

I'd say if there's an animation studio near you, 
why not try to see on some boring day?  Can't 
hurt.