I like coco fiber (bed-a-beast) for display tanks. It holds humidity well, is absorbent, and doesn't have any large chunks that may cause impaction. Also, it doesn't have the same environmental impact as cypress mulch.

However, if you are looking for something for your racks, newspaper is probably the easiest. Here is the website that sells pre-cut tub liners that are more absorbent than newspaper and do not have any printing: http://www.cageliners.net/index.html