I have had similar troubles. I was looking for pure lead so that I could either cast pure lead Minie's, or add components to make a specific alloy for 45-70.
I recently found that Buffalo Arms will sell pure lead, and ship it in flat rate boxes. Just bought some and am waiting to cast.
Given that I can't find any gauranteed source of pure lead, this seems to be a good source, if you are willing to pay shipping.
Can't answer the question about Arsenic.