Bismuth Alloys: Cerrobend which Doug has used a lot, is just one of these alloys. Another one is the specific alloy used to form the beam shaping blocks for radiation therapy called "Radiation Therapy Shielding Alloy - Bismuth", this one melts at 250 F, and it will reproduce pretty fine and thin mold details, when it is fresh. There is a seller selling a bunch of this stuff on ebay for about $3.00 a pound.
Getting the fresh alloy is worth all of the extra effort, since upon repeated re-melting, and re-use, some component metal in the alloy oxidizes, and the Bi crystals which form on cooling get larger, and greater in number, until it becomes very difficult to get decent castings with slow cooling, or with a large mass. This stuff is real useful for so many things, since you are free to use many types of plastic, as a mold material. In the rad. therapy setting they use styrofoam blocks as mold stock. This alloy also has a very low background radiation count, my most sensitive probe sees nothing from a block of it. Maybe Doug has some thoughts about what metal is being lost due to oxidation? This may provide a method to re-fresh the alloy, when it starts to behave more, and more like pure bismuth. The MSDS on at least one companies alloy version indicated that it contained cadmium, which may in part explain why it works so well as a gamma shield for fast neutron probes. Taking this into consideration propper hand washing etc. would be prudent just like for lead.
One thing about these alloys is that when you have it around, you tend to keep finding new, and more uses for it, so on several occasions: What I had thought to be a sufficient stash turned out not to be enough. My father wanted some to do chamber casts in firearms, for which it worked great by the way. Currently there is a seller on ebay who has the fresh rad. shielding alloy for about 3 bucks a pound, see the trading post entry. Thank god for USPS flat rate boxes : )