The only problem with the tire weights is that they are not a "soft" lead and will not pour is some molds. I have used tire weights and probably will not in the future. You can try some of the recycle shops, they may have some lead and it is reasonable. Plumbers lead and some sheet lead is available there, if no one has already bought it.
You will need to check the wheel weights, not all are lead. I had about 10% that were not lead when I melted mine down. They are usually dirty and will take some time and effort to get clean lead. I took half of a day to melt and clean the lead wheel weight that I used.
Make sure you treat your molds with either candle soot or "drop out" if you are not using the soft lead. Best of luck.