Because they are animals, they won't just appear. They have to be introduced to your tank. This is typically done by adding LR. LS might do it too, but I'm not certain on that one.
some sand may have it, our lfs sells sand by the pound, from a tank that is established, which is full of life, i jsut would not try to buy it this way, jsut apound or 2 to seed your bed, and as mentioned, lr does normally carry them too
HTH