I just did the sculpt tonight, if I get around to making the mold I'll make sure to post that as well. Here are some pics!
Here's one site that you can buy it for cheap, this is a 'medium" firm. You can use a desk lamp and some tin foil to heat it up until soft (it gets firm again fairly fast) and because I wasn't doing anything large I didn't need an internal armature, but I did use that foam block and wooden bamboo stick to give myself a better view while I was sculpting.
Anyway, here's the nearly finished amulet, I need to tweak it a bit before molding, but it's close enough to the concept to work well.