I'd also recommend keeping tabs on sites that 
deal in U.S. animation cels.  VanEaton Galleries 
has managed to get some in, in the past.  I don't 
know if they currently have any available now, 
but it's worth checking.
And, as mentioned above, there have been some on 
eBay recently..
It all depends on what you're looking for.
I managed to get a few G1 cels, but it was a long 
time coming, and I haven't managed it again 
since.  It just takes a lot of patience.

~Mala