Tire shops for wheel weights - watch out for the zinc ones, they'll ruin your batch. castboolits.com has articles on separating them out.
If you know anyone that deals with underground communication lines - older installations have underground vaults every so often for splicing, maintenance etc. Splices used to be done in a lead casing (big ones), pure lead - no longer allowed (worker's compensation regs). Any of the older ones now, if they need work, the lead is cut out and replaced with some kind of plastic. Lead gets thrown in "the bin". I got a few hundred pounds from an acquaintance doing this kind of work - best (cleanest) I've ever had.
Be very wary of lead cable that's been used for under water transmission lines - I bought ~250 lbs of this stuff and it was terribly dirty - lots of green / yellow powder separated when I smelted it - someone said it was cadmium - kinda scared me!