If you're confident of its composition, take the local scores where you can. If it's some of the harder "type" alloys (see our forum lead alloy calculator), better still.
I bulked up my own inventory with a lot of range scrap, which I segregate into (1.) jacketed, (2.) shotgun slugs, and (3.) other people's cast.
1 and 2 are very soft of 1% antimony or less, so the "sweetener" mixes are very good to have around to turn that into something more versatile. 3 tends to end up in the vicinity of clip-on wheel weight, but you never really know until you slag each batch down and put it on a hardness tester. A bucket of lino or other "type" will go a long way in improving the soft stuff.