Hi!

There are two TLU cels of the Unicorn available 
for sale on the Rubberslug shop here:
http://www.rubberslug.com/shop/shop_series.aspx?
n=125

There is also a lovely cel of the Unicorn with 
Schmendrick available on anime-link.com, however 
it has a higher price tag. I think in general 
these are the going prices for TLU cels unless 
you manage to look long enough to find a deal. 
We seem to be in a TLU cel lull right now anyway.

Hope this helps! Good luck!