Bio active substrate?

I've been looking to make a bioactive enclosure for my corn snake and I've read that Arcadia earthmix arid would be a good substrate. Unfortunately I can't seem to find any locally and any site I found that will ship to me here in Canada want about $60 to ship it, which is about the cost of 2 bags.

I was asking a friend who works at a local reptile shop(not a big name store) if he sold anything similar and he said that I should just use top/potting soil without fertilizers, pesticides or herbicides.

Would this be safe and would I need a drainage layer? Also would this be ok for other reptiles?

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The Arcadia mix looks awesome, but I bet you could get by with a home brewed mix without too much trouble. For a corn you might not need a drainage layer, but I don't think it's necessarily a bad idea to have, especially if you're wanting to use live plants. I used lava rock for mine, though mine is a paludarium and has a layer of Matala filter pad underneath that. Organic potting soil would likely be ideal, and you could mix in a little play sand and maybe coir? You can even add some azomite powder to increase the mineral content of the soil for any plants you want to add, just be sure to use a mask when working with it dry.