My dirt/soil equivalent for my isopods is organic compost of any brand. Is that what you mean? As in, a single ‘ingredient?’ I’d imagine that any soil that’s organic and free of fertilizers would work.
Or do you mean what folks use for a full bioactive substrate mix? Here’s a post I made about the substrate I use for my isopods. I plan to use something similar in the bioactive I plan to set up.