The main idea is that sand is simply the substrate in which beneficial organisms can live. whether they are present in the sand or not is what separates "live" sand from "dead."
Therefore if you add a small population of that life to a large amount of "dead" substrate, it will reproduce and expand to inhabitat the new territory, thus your sand will slowly begin to "live." Since all you need is a small amount of life to "seed" a large amount of "dead" sand, the notion of buying those big bags of live sand is largely thought of as a waste of money.
you can buy some of the "dead" substrate materials like dry crushed aragonite or limestone, as AM pointed out, and then there are many ways to "seed" it: good live rock, a cup of live sand from an established tank, or even a rag soaked in the fuge for a couple days or some old activated charcoal that has been used in a well established tank.