Hydroton can be found at a specialty greenhouse, garden center or a hydroponics retailer.
I have also broken up unglazed clay pots and used the pieces for the first layer ... hydroton is fired clay.
The charcoal must be that sold at greenhouses or activated aquarium filter charcoal.
From the bottom up their should be a 1 to 2 inch layer of hydroton then fiberglass screen then 1/4 to 1/2 inch of charcoal, screen again, then coco fiber,peat, organic soil mixed or one or two of the three 2 to 3 inches or up to the vents on the front of the enclosure (don't cover the vents).
Usually I place the plants (in the pots they came in) on the last layer of screen and fill in the soil around them.
-Brad