Here is a cheap spec for you. It is a little thin to go on at the moment but comes from a legitimate source. Jason Momoa posted a brief workout clip of himself mentioning “Getting ready for the castle. Watchtower.” Could this be in reference to the Justice Leagues headquarters floating in space?
The Watchtower first appears in Grant Morrison’s run, JLA #4. There are a few copies on eBay, including a 9.4 CGC starting at $17.99. Mycomicshop has copies of it in VF for under $3.
Now I know some people will say that I am crazy for mentioning this book because it is the first appearance of a place and not a person, but I have seen it happen before with Knowhere appearing in Guardians of the Galaxy.