I know; so annoying! I've seached high and low, and only found a partial copy in Google-books. I've found many editions of Perotin, on CPDL & IMSLP, but not the Leonin. I've heard many recordings, so there must be a score somewhere. Maybe.

The staff at a university music library could probably answer this. Some works are published in academic journals or music book series, if not in conventional performing editions.

I just double checked my CDs.

Track #57: Leonin, Viderunt Omnes, 8:13, Concentus, Paul Elliot, tenor and director.

I have that book here somewhere.

This is the link to the score book.

http://www.amazon.com/gp/aw/d/0393979903/ref=aw_1st_sims_1?pi=SL500_SY115

I can't search here at home to find the book today, due to a temporary back outage. However, I can state that the scores in the Anthology are rather small and they are reproductions; so if you can find where the editors reproduced it from, that might be a better-sized version. Failing that, the Anthology's version may suffice.
Note: the contents of this textbook change from one edition to another, so buying an old edition may need some care to make sure it has the work you want.

Try GregoBase.

If anyone is interested, there is a digitization of a MS of the Magnus Liber Organi at IMSLP.

I have recently purchased a copy of a book entitled "The Rhythm of Twelfth-Century Polyphony." It has a huge section of transcriptions from the Magnus Liber Organi. If you can get your hands on a copy, it will have additional material in it. I found mine for around $30.

Thank you for posting that resource; I will definitely be getting it.