The main thing to watch for when you make molds is to verify that the silicone that you're using is food-safe. If you're shopping at a craft store there are brands that are and brands that aren't, so make sure that you double-check that.
There are some online sources for mold-making materials, www.makeyourownmolds.com being one of the largest that I know of. There's also chef rubber, which also has equipment and sells premade molds, not just the materials.
|See, they do look alike!|
Price-wise, the best bet for small amounts of this is to buy it with the 40% off coupon at the craft store. If you want to order large amounts you can get it directly from the Amazing Mold Putty company in bulk on their website.
The Make Your Own Molds site has a discount if you order larger amounts of the silicone, but unless you want more than 4 pounds of it it's not a huge savings.
Or you can go to my etsy shop and order custom molds. That's a lot easier.
On Friday--Making a one-piece mold.