Many people making vacuum formed molds for chocolate prefer to use PETG plastic instead of the more expensive polycarbonate. Polycarbonate is definitely the gold standard when it comes to professional chocolate molds, but it is probably not necessary if you are only going to be making short production runs for your own purposes. It is much easier to form and yet is still very strong.
According to Sheffield Plastics, the manufacturer of Vivak (PETG),
"VIVAK is the brand and market leader for PETG products. Among its advantages, VIVAK PETG products offer superior impact strength over acrylic and cost effectiveness compared to polycarbonate, and offers deep draws, complex die-cuts, and precise molded-in details without sacrificing structural integrity. It die-cuts and punches easily, and can be bonded or fastened with adhesives, ultrasonic welding, or rivets. In addition, VIVAK is easily decorated by painting, silk screening, or hot stamping. Easy to fabricate, form, bond, and decorate, VIVAK PETG sheet products are well suited for a variety of point of purchase and other applications."
On their website, they have a 12 page guide for fabrication, forming and finishing. To view it click here: http://www.sheffieldplastics.com/web_docs/BRO009.pdf.
Chocolate molds made of PETG are usually made in thicknesses of .020", .030", .040", and .060". Obviously, the thicker the plastic, the more rigid the mold will be. All of these thicknesses, with the exception of .060", are easily cut with a good pair of scissors. The .060" is a bit more difficult to cut, but it can be done with scissors, scored and snapped, or with a saw.
There are many sources for buying PETG over the internet. Just type "PETG" into any search engine and you will find plenty of companies selling it. Also, I advise you to check your local phone listings first because plastic is fairly expensive to ship, as it can be quite heavy.