You want zinc? Hit Home Depot - Just picked up a roll of "moss-repeller strip" - It's a roll that claims to be 100% zinc, like narrow flashing, 3-4 inches wide, intended to be used kinda like flashing and be weathered away to "contaminate" and/or be absorbed into whatever material your roofing is, killing/preventing moss, supposedly for years between applications. I can't personally vouch for how effective it is against moss, since what I'm using it for is to wrap the edges of wood doors and similar places that the livestock might take a hankering to gnaw on. But if you're wanting zinc for alloying, I'd think this would do the trick at a reasonable price - The roughly 6 pound roll cost me something like 7-8 bucks - as well as being a handy way to set the "dose" for the pot - not so big as to be a pain in the arse to handle or store, easy to weigh up for specific alloy ratios, and being so thin, I expect it would melt almost instantly when you drop it into a pot of hot lead.