What are you casting for, makes a difference to how hard you want them..
Keep notes of hardness. [Making your own hardness scale is not difficult, a center punch a dedicated weight or hammer and choose a distance to drop] measure the divot width with calipers... Small is hard, big is softer.
Tin is a easy alloy to firm up lead, [notes] ... Linotype is a bit harder and slightly lighter. [gets bullets hard enough to spit noses if hit with hammer rather than mush down.]
Stained glass shops are a source for near dead soft lead, a little solder will be in the scrap. {near free}
Tire shop lead is hard, modern gun bullet hard. (Little bed of coals, second hand store pot and metal spoon and muffin tins, make great little ingots.)