Notebooks are scattered around Stray in The Slums area, and you’ll know one is close by when you see the sign of The Outsiders. The symbol is two dots above a small, incomplete circle with a gap at the bottom.
Whenever you see that symbol, look around and you’ll find a notebook.
Where To Find Momo’s Notebook (1/4)
You’ll obtain Momo’s Notebook as part of the story. Once you meet Momo, show them the first Memory, which is a picture of the beach, and they give it to you after your conversation.