Wikipedia has a list of all of the volumes
https://en.wikipedia.org/wiki/Man-Kzin_Wars and you should be able to use the ISBN numbers from there to accurately find any volume.
As far as I know very few of Niven's back catalog have made it to eBook including the first volumes of man-kzin.
In fact if you go to the author page at
http://www.baen.com/author_catalog.asp?author=lniven you can find those early book available in dead tree versions but not ebook.
Also, I do not think there is a single omnibus of all of the man-kzin in either eBook or dead tree.