For my BP I use ESU Lizard Liner substrate - it's a green felt like carpet. I love the way the color green looks against a Ball Python.
Shredded aspen is a popular choice. And of course there's newspaper. The compressed coconut bricks (Bed-a-Beast, EcoEarth, etc) are great as well - especially for humidity.

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Check Out My Albums
1.0 Ball Python "Aragorn"
1.0 Kenyan Sand Boa "Gimli"
1.0 Saharan Sand Boa "Frodo"
0.1 Rough-Scale Sand Boa "Arwen"
0.1 California Kingsnake "Gentoo"
1.0 Mexican Black Kingsnake "Indigo"
1.0 Snow Corn snake "Chile"
0.1 Australian Cattle Dog/Pointer
"Kira"