The best places to look are on Yahoo Japan and
Mandarake. A LU cel will usually show up in one of
those auctions every few months. 

There is currently a cel of Amalthea sleeping on
Mandarake. The price at the moment is only about $30

Cels of the Unicorn tend to run a bit more. The
prices have come down a bit. There was large image
of the Unicorn on YJ a few weeks ago. I didn't see
the ending price, but last I looked it was only
about $300.