Let's hope that foundries step up to the plate and begin offering wheelweight ingots for sale. In the meantime here are some suggestions on how to brew your own wheelweight alloy from common foundry alloys.
For all of these homebrew recipes you may need to add a spoonful of lead shot to boost the arsenic content, though arsenic is a naturally occurring "contaminant" in most lead alloys so it may not be necessary unless you are using 99.99% pure lead.
Also note that while we think of things like stick-on wheelweight, lead sheeting, lead radiation shielding, and lead pipe as "pure" lead, in most cases they actually contain 0.5% - 1.0% antimony and also a little arsenic, so that works to your advantage if you use them for the "pure" lead in these recipes.
3 parts pure lead
1 part linotype
spoonful of lead shot, if needed.
= 1% tin, 3% antimony, and 96% lead
1 part pure lead
1 part hardball alloy (92/6/2)
spoonful of lead shot, if needed
= 1% tin, 3% antimony, and 96% lead
You can also buy antimony or antimony/lead blend from Rotometals, then add it to pure lead. Pure antimony will not melt in your lead pot though. Not sure if you can melt it with a propane torch while it floats on top of the pot? I'll have to try it sometime.
As for whether or not you need to add lead shot, that is easy to determine if you have a hardness tester. If your heat treated bullets are 21 - 30 BHN then you are good to go. If they're not as hard as normal heat treated wheelweight then add a spoonful of lead shot and try again. Only a trace amount of arsenic is required. The arsenic content of lead shot varies quite a bit and the manufacturer won't necessarily tell you how much it contains, so trial and error may be required. Other additives like sulfur can be substituted for arsenic as
discussed in Wiljen's article.