Jet Force Gemini

The item in question is located in the Thorax section, actually.

Now, the magazine isn't available the first time you go to Eschebone as Lupus, and actually requires Vela (which in turns requires the player to have beaten all Gemini members' prologues). When you went through as Lupus, you probably remember a cavern with a pond and health upgrade; Vela can swim down into a secret entrance in that very pond. About three rooms in will be a container that contains the magazine.