Like Bob said, try some of the smaller mom and pop type places. The small out of the way, repair garages that do tires. When I walk in, I always smile, and offer to BUY their wheel weights. I never ask for free wheel weights any more. By offering to buy, you set them at ease. I usually throw a price per bucket at them, and $10-15 isn't too low, but usually end up with paying about $20 a bucket. You need to be flexable. Then, I bring out my 6 gallon buckets (they were expecting small ones). . . I usually score 250 pounds per bucket, which is about all this old guy can lift these days. Lead, in the London Metals Market is about 13 cents a pound right now. So, when you factor in the steel clips, tire stems, etc.... be friendly, and even offer to bring donuts or lunch on your next trip. That will usually open the door to future opportunity.
The local place I get most of mine is the same place where I get my tires when I need them. I have been smoozing the owner for years, starting with small amounts that he gave me for free. Like small boxes. I moved up to bigger quantities, usually for $5. My previous time there was six months ago, and I got a six gallon bucket for $20. Last week, I scored his WHOLE BARREL out back for $100! Some 2,000 pounds of wheel weights. Pay Day!
If you set up a string of small garages, and get 50 pounds (which is a mere drop in the bucket, as they say) from each.... ten of those is 500 pounds in a lead run! Do that every month, and you never run out of lead.
Maine joined the get the lead out of the environment (Thanks, California!) movement two years ago, so lead wheel weights are slowly going away, and only exist on older cars where they get new tires.
I can't believe how many places I've been to that already have someone buying from them. I'll be out checking a few leads myself this weekend. I never thought putting my hands on lead would be so hard after reading about how others were doing it on Cast Boolits and GlockPost.