Pro's - Lightweight, Strong, Durable, sandable/paintable, found in most hardware/automotive stores.
Con's - Messy, harmful fumes/dust, $30 a gallon, not good for beginners.
You can find this stuff in most chain hardware stores like Home Depot, or Lowes in the adhesives aisle, or look in auto repair stores (Pep Boys, Auto Zone) in the body repair aisle. It's usually called Fiberglass Resin, or Polyester Resin. The latter is more accurate, as the fiberglass is actually sold seperately in cloth or mat form.
It's a two part plastic resin. You pour some of part A into a mixing container, then add a little bit of part B - the hardener. Mix well, and brush or pour it over your pieces/molds/whatever. IF you have mixed it properly the liquid plastic will harden in 20-40 minutes. Various additives can be added to change the properties of the resin such as dyes, thickeners, and fillers. The fiberglass cloth is added by applying strips of the material into in the resin in layers, after the first coat has cured, then brushing more resin overtop of the cloth. The fiberglass cloth is what makes the plastic strong, as polester resin is quite brittle without reinforcement.
This material is not for beginners. It is a hazardous chemical, and can cause skin irritation, chemical burns, dizziness, nausea, nervous system damage and other fun stuff if it is not handled EXACTLY as the directions indicate, which means that you will need a Respirator mask (one of the expensive ones that filters out the fumes), plastic gloves, eye protection. Adequate ventilation is an absolute MUST for this, and if possible it should be done outside.
Dude you aren't buying cardboard are you? cause you know you can go out back behind like any store that recieves things packed in big boxes ( HomeDepot, Lowes, Furniature stores, Craft stores and the list goes on. )
They all just toss out the boxes ( FREE CARDBOARD )