From what I've read its basically just buying them separate and mixing it yourself at home. It saves you going out and actually scooping it up yourself. Homedepo or Lowes sell the playsand, dirt, and compost (dead leaves or what not). You'd have to pick up the rotting sticks yourself probably which has most of the organisms that would serve as your cleanup crew. Just be sure anything you get has no chemicals or fertilizers in it.
Hope this is somewhat helpful
