It's not a substrate, it's a carpet. I got it from Petsmart, though they're sold at almost every pet store. I highly recommend it simply because it's easy. You can take it out and wash it then throw it back in. It's not rough on your snake's skin and most importantly: it doesn't get stuck in your snakes mouth when it's eating.
I used to use aspen bedding but when I would feed her, she would swallow some chips. It's dangerous because it can get stuck in their mouths and cause infections. So I switched to carpet immediately!