A good source for lead is often shooting ranges. The backstop lead can be smelted down and recast as ingots to make new bullets. This is often VERY dirty lead so it's best to do it in a dedicated smelting setup such as a turkey fryer and a metal pot, outside. Many trap/skeet ranges also have reclaimed lead shot for sale. Shot is a good antimony source, too.
I batch my range scrap ingots out of 60# pours to homogenize the lead alloy as much as possible. I find that the BHN (hardness) of my range scrap tends to be about 6-8, which is halfway between pure lead (3-4) and wheel weights (10-12). My source allows cast bullets and there's a lot of them shooting there.
Some ranges don't allow cast bullets so you will probably find a high amount of nearly pure lead from .22 rimfires and the cores of FMJ or plated bullets. This lead will tend to be quite soft and might need to be alloyed with something else to get the hardness up.
You could also try scrap-metal dealers. If there are any stained-glass shops or radiator repair places it's worth asking. Call the recyclers to find out what they pay, and be prepared to pay the same or a bit more.
Tire shops are pretty hit and miss, mostly miss. Nearly all the major shops (Wal-Mart, Firestone, Goodyear, Tire Kingdom, Tire Discounters, Pep Boys, etc) will have a contract with Johnson Controls or Exide Battery to return all used weights with the battery scrap. They won't let you buy or beg any lead. The people to ask are the independant shops- a lot are under contract, a lot aren't. I find that a shop that you do business with is more likely to let you get lead than a shop you don't.
I've had much better luck cruising by the tire shops right at opening or closing time when they are not busy. I usually bypass the front counter and ask the shop manager or lead tech directly, usually with a couple of $5 bills in my hand. I've also learned not to mention bullets and instead if asked, I say I make fishing weights. This is because there was a surprising number of people who when they heard BULLETS became unfriendly or even hostile in one case.
Something about tire weights- California banned lead wheel weights. Many manufacturers and distributors, rather than maintaining dual inventory systems for CA and Not-CA, decided to just not use lead anymore. Over the past year the amount of non-lead weights I find in my weight buckets has steadily increased. Instead of lead, it is common to find zinc and iron weights and these are useless to a bullet caster.
One thing to know- be very cautious of buying lead ingots from anyone you don't know, especially on eBay.
Once lead has been ingotized, you have no way of knowing what the actual source is.
Why is this a problem?
When I buy wheel weight lead, I want 100% wheel weight alloy. This will typically be between 3% and 5% antimony and slight tin.
If I want pure lead, I want pure lead.
Some people melt down the entire bucket of wheel weights without removing the stick-on weights. Stick-ons are nearly pure soft lead and this dilutes the antimony content.
Some people may even melt down the wheel weights then bulk it up with whatever other lead they can find so they have more "wheel weight alloy" to sell. And that's technically not incorrect- it's still a "wheel weight alloy", but it has less antimony.
This deceit is especially common when talking about type metal alloys since these contain a higher percentage of antimony and very expensive tin! I personally will not buy any alloy claiming to be linotype, monotype, or foundry type that has been recast into ingots. I want to see it as type metal so I know that's what it is. This is still no guarantee since many type shops recast their alloys. But seeing type metal as letterpress is more a guarantee of alloy composition than whatever claims are made about their ingots. I honestly don't understand why a seller would smelt down letterpress into ingots anyway- typically, letterpress packs very well in boxes as is and you don't need to add a whole lot to a 10# or 20# pot to get a good tin level in the alloy.
There was one specific case in which a fellow was casting wheel-weight alloy ingots and then water quenching them to attain an increased hardness and then selling them as type metal ingots (which are harder). That meant he could about double the price of his ingots.
So why does this matter?
Antimony in lead bullets is useful to toughen up the lead. It allows hardening by water quenching or oven tempering. Tin is needed to make the lead flow better. Antimony by itself is difficult to alloy at home. Tin is easy to alloy, it's just very expensive compared to lead. A typical casting alloy is the HardBall alloy, 92% lead, 6% antimony and 2% tin.
So, long story short, I would rather buy raw wheel weights as wheel weights than ingots claiming to be wheel weights. That way I can know it's all clip-on weights. And I can save the stick-ons for other purposes. The same goes for type metal.