for my hatchlings indoors their first year (I don't think it's successful to keep box turtles indoors other than that, or possible for winter if you can't hibernate them), I use a mix of topsoil and bed-a-beast, or else a mix of top soil and sphagnum moss. I will take the bed-a-beast or the sphagnum and soak it thoroughly, then wring out handfuls of it to crumble over the soil, then I mix it in. It ends up being quite inexpensive and works well for me. One brick of bed-a-beast, for instance, is more than I've gone through in three years of indoor cage with a forty breeder. The brick expands A LOT when you soak it, just be sure to ziplock or otherwise seal and store the leftovers.