Substrate is a very personal choice. We each have our own favorites that we 'think' provides best for what the turtle needs. My personal choice for box turtles (well, mine live outside, but for indoor babies...) is potting soil (without perlite) and a thin layer of small bits orchid bark over the top. I just buy generic, or store brand, potting soil, either from a big box home improvement store of my local hardware store.