They don't show up as often because they are keepers. But if you take your time and search, you can find one. I have not seriously listened to the LS26. IMO the LS27 is not one of ARC's better efforts.
I see a couple of Ref 5se preamps going for $7000 here on Audiogon. That is a deal at almost half price. The Ref 5se is so very much better than the LS27 it is amazing. It is very special and another one that will be a keeper. There are a lot around because people are upgrading to the Ref 6. But I believe they may also become hard to find.