Google is an incredible tool, despite my misspelling perlite. I just copied and pasted a pretty good description of what perlite is. There is a pretty good description of its properties and uses. Hope it helps. You can buy perlite at Home Depot, Lowes, or any other garden supply center. I don't believe it's expensive at all - I didn't take note of the price, so it couldn't have been bad. See article below.

Perlite is a unique volcanic mineral which expands from four to twenty times its original volume when it is quickly heated to a temperature of approximately 1600-1700 degrees F. This expansion is due to the presence of two to six percent combined water in the crude perlite rock which causes the perlite to pop in a manner similar to that of popcorn.

When expanded, each granular, snow-white particle of perlite is sterile with a neutral pH and contains many tiny, closed cells or bubbles.

The surface of each particle is covered with tiny cavities which provide an extremely large surface area. These surfaces hold moisture and nutrients and make them available to plant roots. In addition, because of the physical shape of each particle, air passages are formed which provide optimum aeration and drainage. Because perlite is sterile, it is free of disease, seeds, and insects.