Wheel weights. The softest (and least expensive) lead-tin-antimony alloy is wheel weights. Wheel weights are reasonably consistent and are the favorite alloy of many modern reloaders. They produce an alloy that casts well. Some bullet casters add tin to improve the fluidity of the melt and increase the hardness. I have cast bullets from wheel weights with one percent added tin, and I have added soft lead to reduce the hardness of bullets. The added tin did not improve the quality of the bullets. Adding soft lead greatly reduced the quality; many of the bullets did not fill out completely. I use straight wheel weights to cast the bullets for my percussion Sharps carbine and rifle, for 45 Colt bullets, and for bullets for a cap and ball revolver.
Wheel weights can be purchased at a very low price from tire stores, or they may give them away. Unfortunately, they come mixed with dirt and other junk. If you cast bullets with wheel weights, melt the wheel weights outside the home, unless you don't mind moving out of your house for a day or two afterwards. Remove all of the dirt and debris from the melting pot that you can and cast the metal into ingots for later use. Use a fluxing preparation as you melt the weights to reduce the need to flux while you are casting bullets. (See Section 7). When you melt the ingots later for casting bullets, they will produce almost no smell.

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No. 2 alloy. Many casters of bullets for modern and black powder firearms use No. 2 alloy. It was originally a mixture of 90 percent lead plus five percent tin and five percent antimony. Today, an equivalent alloy (producing the same hardness bullet) will have more antimony than tin. For several years, I purchased (at considerable expense) No. 2 alloy from a reloading shop and used it for the bullets for my breech-loading firearms. Eventually, I found No. 2 alloy had no advantage (other than convenience) over wheel weights. (Too soon old; too late smart.)
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Linotype alloy. The hardest of the alloys described here is linotype alloy. For many years, linotype alloy was purchased by reloaders in the form of scrap linotype and made excellent bullets for modern cartridges. The linotype alloy had enough tin to help cast bullets that otherwise would not cast well (very long bullets). But linotype is about as common as a 1950 Ford today. Nevertheless, some bullet casters still buy linotype lead or an equivalent alloy. It probably is harder than required in black powder firearms, but some black powder shooters use it with good success.