I recently started using coco fiber stuff (like the bed-a-beast, coir that comes in compressed bricks). It holds humidity well, and I get it -very- cheap at my local hydroponics store as opposed to the pet store prices on tiny blocks. One downside is that whenever my big pastel girl knocks over her water bowl there's an overly wet part of her tub, but it's been an easy fix by getting a different water bowl, haha
Edit: I have no idea why it did the thumbs down thing for my reply
Edit 2: Nevermind, figured it out!