I do multiple castings of my Santa's. I use 'Durham's rock hard water putty'. Its mixed with water, and its an old, time tested casting material. It comes in large or small quantities from your hardware store. And for the mold I use 'Abatron's MasterMold 12-3'. Its a non-toxic polyurethane flexible liquid paste, when dry, can be used over and over. It can be found in some woodworking catalogs or see the link below. All come with directions on use. Although, there are MANY products, these are easy and fun to use. Below are links for your research.
http://www.waterputty.com/casting.htm http://www.abatron.com/