Try your local radiator shop. If you're lucky they will let you take the solder drippings they have cleaned from the bottom of the tank. It's usually 50% to 60% tin. Got a lifetime supply on my last run, 3 five gallon buckets of the stuff. It is messy and has a lot of crud mixed in. So far I've found 4 screw drivers, 2 needle nose pliers and various other trash. Smelt it outside, flux it well and be careful of the fumes.