Lately, I've been getting lead products from the local scrap yard. They have the same problem with wheel weights, though. Too much zinc. Last time, I bought a bucket of wheel weights, I ended up with the same issue you had. Not even worth the time to sort through it all. The scrap guy did tell me I could sit there in his yard and cull out the zinc but still not worth it.
They have a lot of raw lead sheeting, though, so if you're willing to alloy up with tin and antimony, you can get some really good deals. One day, I found a bucket of old bullets someone had cast but they were in bad shape and had to be melted down. Some decent lead, though.
Good luck to you.
--Wag--