Yes, silicone is expensive, and food-grade silicone is even a little more expensive. Vacuum formed PETG molds are much, much cheaper.
(Especially since I’m thinking of situations where you need to produce a bunch of chocolates—say, one or two per person at a large event—so you need a bunch of molds to do it in reasonable time.)
The best place to get sheet plastics is usually a local plastics supplier, so that you don’t have to pay shipping. Some have a $100 minimum order to keep the riffraff out, so you probably need to look up plastics suppliers in the yellow pages and call around. Try to find one with no minimum, or a low minimum, or who’ll waive the minimum for “will call” orders (that you come in and pick up yourself).
A full 4 x 8 foot sheet of thin PETG should be in the low tens of dollars, and they’ll roll it up so it’ll fit in your car. (Be sure to ask for FOOD GRADE PETG. Other PETG may have various additives in it that are not good in food.) You can take it home and cut it yourself by scoring it with a razor knife or other very sharp knife and snapping it at the score.
(Actually, for thin PETG you can usually just cut it with scissors.)
You can cut a 4 x 8 foot sheet into 24 12″ x 16″ sheets, which is a convenient size for lots of stuff, and fits very comfortably in most home kitchen ovens. Those smaller sheets end up costing around a dollar each, and can have a couple of candy-bar molds formed into them, or a bunch of smaller molds.
If you just want to make one candy bar-(or belt buckle-)sized mold, and don’t want to buy (or break open) an expensive silicone kit, you can get food-safe silicone putty in small quantities from a Hobby Lobby craft store.
They sell something called “Amazing Mold Putty” which is a two-part putty with about a three minute working time. It normally costs $20 for 2/3 lb., but if you wait for one of their regular coupon specials, you can get 40 percent off, so it’s about $12, or $1 an ounce. Not bad for food-safe silicone, and it should be enough for a belt buckle mold or something comparable.
Check the Hobby Lobby web page for their weekly coupon. Every few weeks they have one for 40 percent off any one item in the store. You have to print out the coupon and take it in to the store to get the 40 percent off, but you can print out several and use one a day for a week, and get 7 things at the 60 percent price in a week. If there’s a Hobby Lobby on your daily route, that can be a cool deal.
As I write this, on July 10 2007, they’re running that deal. Here’s the link to the coupon to print out:
Even if you’re doing vacuum forming, that can be handy stuff to have around, for making intermediate molds.
You can also use alginate, which is cheaper, for temporary molds. (Alginate is like a super jello that you mix with water. Dentists use it for casting insides of people’s mouths, and lifecasters use it for casting people’s faces, body parts, etc.) You have to use it right away, because it shrinks as the water dries, but you could use it (or silicone) to make a few copies of a shape to vacuum-form molds over. (So that you could put, say, three belt-buckle molds in each vacuum-formed sheet.)