If you wanted something completely bug free Rubstrate would be an idea but I have no idea how it works with snakes. I use horse bedding like megazorb and auibois.
All natural substrates may contain bugs, but there's aren't necessaryly snake mites, they could be substrate bugs.
i know one of ours got some sort of bug from his substrate, we took it to be mites and treated it just as u would mites. now we bake the chips before putting them in. was not fun getting rid