The inside pages can be made from almost anything. This is a short list of ideas to get you started:
Tea/coffee dyed paper, old school exercise book paper, text book paper, vintage ephemera papers, such as receipts, letters etc. Junk mail, advertising brochures and leaflets, scrapbook paper, wrapping paper, wallpaper, paper bags from the bakery or supermarket, calendar pages, digital papers, watercolour/drawing paper, parchment or wax paper, children's book pages and puzzle books.
You can find lots of videos on youtube for inspiration. I've linked a few below for you to look at.
Happy crafting, Melanie.