Lead with gold metallic paint. Gold and lead are only like one atom apart, but you can find lead cheap or free. It also has a very low melting point, in case you want to cast your own bars.
If you go out to buy it, it isn't that bad. But, there is another route... Drive around town, and hit every garage and tire store. Ask them if you can scavenge some of their old tire weights. Like I said, it has a very low melting point, and you can build a box frame, fill it with sand to make your molds.
They wouldn't have the same heft or sound, but if you just need the visual aspect, like a stack of bars in a vault. If that is good, pick up one of these, and use it as a master to mold more yourself. They are pretty.