Before you can cast bullets, you need clean lead in a usable size, at a known hardness.
Lead is often gathered by working a berm at a range, gathering the bullets that collect there over time. Those bullets are dirty, mixed lead, some with copper jackets.
Another source is tire weights. The alloy that tire weights are made from makes pretty good pistol bullets. They are also dirty, with metal clips. When you get a bucket of weights, it has cigarette butts, air valves, and unfortunately, some newer weights not made from lead.
Sometimes large blocks of pure lead can be found. Medical lead blocks for shielding radiation, old sailboat keels, old plumber's lead are all possibilities.
Pure lead is very soft. People add various tin or antimony blends to make the lead harder. Sometimes casters use linotype, a lead alloy that was used in newspaper printing until the 1980s, to make a hard alloy. If you can find some linotype, it is a known hardness. It can be used by itself or mixed with pure lead in known quantities to make consistent alloys.
You have to melt the lead, clean the melted lead of the debris, flux it and remove the dross, then pour the ingots. Safety is a huge issue. Lead melts around 620 degrees. any contact with the lead results in a burn. Lead is a heavy metal, ingesting the lead in any way is serious health risk.
And the end product is only an intermediate step in casting bullets.