Not being a purist, I just use whatever cactus mix is available (usually Miracle Gro) and add a lot of pumice, maybe 30% or more (depending on what I'm potting). You can substitute perlite, not my preference but perfectly fine and usually more easily available. I think snakes can take more organic matter that some other succulents, but the goal is to achieve excellent drainage that allows all excess water to freely drain through while retaining what the plant needs —evenly distributed throughout the media along with oxygen. This is different from how peaty media tends to accumulate water at the bottom of the pot where it gets soggy and anaerobic (no oxygen) and rots the roots. This also allows you to water completely and more frequently without fear of overwatering. Don't forget to fertilize, lightly and infrequently. Every potted plant needs human intervention for nutrients, even if they are content with only a little snack.
