Lead may seem like a good choice, will easy availability and low melting point, but be sure to melt it in a WELL ventilated area. You really don't want to get lead poisoning from the fumes, or even from general handling. This is the main reason I would use lead-free solder.

This is a problem I've been brainstorming in the past couple of days. I've had good luck in the past casting parts with solder in plaster of paris molds. These were one-sided molds, so a sphere would be much harder and beyond my experience. Basically, I mixed up some plaster of paris and poured it i...