Bill mentioned the dimpled Kraft paper. You can get it at Uline.com. It looks like very thin cardboard in color with about 6-10 dimples per square inch which gives it texture for the snake to crawl.

Bark in Boaphile or other similar cages will drive you crazy when you clean cages. Aspen would be better if you want to stay with a particulate substrate.

Home Depot sells a package of packing paper that is light gray in color. It is essentially unprinted newspaper. It works well but is very smooth. I crinkle it up some to at least give it some texture. It works well, but I prefer the dimpled paper from uline.