The key to picking up a substrate when setting up a tarantula terrarium is to know the species and to know where it comes from. Ultimate importance, if a desert species, you want to stay away from anything that's humidity loving, you want to move something more to a sand product. There's a lot of them on the market, there's a lot of different colors, that's what you need. Sometimes we mix it in with something a little more like a jungle mix here, there's a peat mossy type mixture that give it a different loamey texture, so the animal can dig into it, all depends on the particular species. If it's from the tropics, if it's humidity lover, you have to move to a wood bark.
There's cypress mulch, there's repti-barks, small chunks of coconut, shredded coconut, it goes on and on. If it's brown pretty much it's a wood product, and it will hold moisture, that's a good way to remember what to do. Anything on the sandy side, and there's a number of different types, again, for a desert animal. Regardless of the substrate, tarantulas need to hide. Certain tarantulas will pick an area of the tank, and make a web type of a nest, others want to burrow and go under something. If this depth of the substrate isn't sufficient, and the animal can't get down deep enough because of the bottom of the tank, simple solution is to put in a hide spot.
These are used primarily in the pet trade for snakes and such, but there's a variety of decorative ones and functional ones. A little on the antiseptic side, easy to clean, a little plastic box with a little gateway in there, the tarantula will make its nest inside. For the humidity lover, sometimes I'll put a clump of moss inside and wet it down, so it will stay particularly humid inside this enclosure. They have beautiful wooden ones that have a corner piece to it, so you can put it in the corner of the terrarium, and all kinds of exotic hardwood hides that allow the animal to go under and be safe. You want to make sure something sturdy, understand how big your tarantula's going to get, don't buy it just for the size you have, but it for its adult size, so you don't have to replace it halfway through.
Substrates and hides are essential, if you make it deep enough, you can push this down into the substrate to make the height of the hide vary. That's all you need to do, just set up the substrate and hide in the tank, just know what tarantula you're dealing with for starters.