Can you use perlite or vermiculite as substrate? It seems like either of those would meet your needs--they're attractive (white and pebbly), sanitary ("popped" volcanic rock), easy to see poops, hold moisture extremely well, and readily available in garden centers (cheap, too). Lots of gardeners use either for starting seedlings, which requires a moisture-retaining, sanitary medium. If in doubt, you can always heat it in the oven to sterilize.

But someone else will have to answer as to the suitability of them with snakes. I can't think of a reason it would be a problem, but I'm a noob with snakes, so let someone else chime in on that.