According to Lyman Cast Bullet Handbook that you might want to acquire, pure lead has a hardness rating of 5 and wheel weights a hardness of 9, but that will vary, so you probably wouldn't want to add pure lead to wheel weights which would lower the rating of the mix even more. By way of comparison, linotype, which has the highest rating, is around 22. If your sources are pure lead and wheel weights, consider just going with wheel weights. When melting the weights, any hardness material like tin or antimony will come to the surface so don't skim that off and try to keep it stirred within the mix.
What is called Lyman #2 alloy is often used for handgun bullets and has a rating of 15 from the addition of tin and antimony. But wheel weights can get the job done and are commonly used.
Another possible source is expended bullets from a firing range. They clean out the lead from time to time and may be willing to sell small amounts at a reasonable price. But sort it and clean it up extremely well to find any live rounds that might have found their way in.