The set is from 1997, so it will most likely be VERY hard to track down. I was lucky and found it on Ebay, but I've only ever seen it the one time. However, I have seen some of the individual CD's sold at Book-Off (a Japanese used bookstore). So, you might be able to get them that way, just no box, or book...
My opinion on the set, is that it's just ok. Sure, there are tons of tracks, but many of them seen hastily thrown together with a loose theme, such as "Under the Sea" which is mostly songs from the "Sebastian's Party Gras" and "Sebastian" albums. There is some stuff on there from LP recordings, so that might be a little harder to track down, but nothing on the set hasn't been released before in some form.