Not sand, repticarpet, or aspen. Aspen in itself is fine but since most people owning a ball python for the first time get the petsmart glass tank hobby kit, aspen can be a little dry to keep humidity up.

Your best choices are cypress mulch or coconut fiber. Zoomed, Zilla, and Reptile Prime are the three popular sellers of these items. Zoomed and Zilla can usually be bought at petco or petsmart.