1 gallon zipper bag of the same high-quality, chemical and fertilizer-free Perlite we use for Orchids, Aroids, Carnivorous Plants, and many other tropicals.  Perlite is a volcanic rock that is "popped" under pressure, very much like popcorn!  It's lightweight, resists crushing, has lots of nooks and crannies, and is amazing for lightening soil mixes and providing aeration at the root zone, as well as allowing for consistent drainage to prevent root rot.  We use it in virtually every potting mix for our plants!