On the lookout for a LU piece for my better-half (and for myself as well!)...it's sorta something we met over, so finding one would be pretty awesomesauce...
They show up every once in a while on Yahoo Japan or Mandarake.
On YJ, the seller may not know the name of the show since I don't think LU has ever been aired in Japan. Considering how much of it is metaphorical and all its odd cultural references, it may be nearly impossible to translate well. Anyways, it is best to look under a general heading of cel since looking only under LU will cause you to miss one every once in a while.
I do have one LU cel open for offers on my Rubberslug site, (Rodenticide 101), though it does not feature the main Unicorn. You may also want to check with the owner of the Unicorn Gallery on RS. I seem to remember she was selling a few LU sketches not too long ago.