1) I know that some people on here have been able to purchase it at walmart but my local walmart doesn't carry it so I had to get it at my petstore (shock, horror on the price tag). I would recommend looking at a feed store though - you might be able to get it cheaper there.
2) No, you don't have to treat it.
3) I've only just started using it, but haven't noticed much impact on humidity. Someone who's been using it for longer will be able to answer this one better than I can.
0.1 Ball Python, 0.1 Creamsicle Cornsnakes, 1.0 Amelanistic Cornsnake, 1.0 Ghost Cornsnake, 1.0 Motel Amelanistic Cornsnake, 1.0 Okeetee Cornsnake, 0.1 Striped Amelanistic Cornsnake, 0.1 Silver Phase Miami Cornsnake, 0.1 Sunglow Cornsnake