If you've got a good plumbing supply shop in town they should have 50/50 bar solder in 1lb blocks. That's generally my source. I've worked with pewter craftsman who use a variety of other elements (Antimony, Silver, Copper, Bismuth) in their cast products, so you could be grabbing a "box of chocolates" when you melt your Goodwill purchases.
When I'm casting bullets I like my Monday bullets to weigh the same as my Saturday bullets. With unknown alloys that may be difficult.