If you have to buy tin it can get pretty expensive.
I found the most convenient source to be common solder, the 50% tin/50% lead variety marked 50 Sn/50Pb.
If you have to purchase new solder, that can get pricey.
Another alternative is to find an electronic surplus place that salvages lead solder from printed circuit cards and equipment. Electronic solder is high in tin content, typically 67/37 tin/lead alloy.
For most of my bullet casting needs, common auto wheel weights melted down and with sufficient tin added to make molds fill out completely work fine.