copper slug

plumbing fittings, heavy guage ground wire, old weather vanes, copper nails, old standing seam copper roof panels, old copper rain gutters, etc. Check some salvage yards in your area. I assume you want small granular peices for ease of compaction? There is the problem, how do you grind the stuff up?

Ebay, look under oxygen free copper rods. You will find assorted widths, buy a rod and cut as many as you need for pennies a slug.

Could be wrong, but I think copper grounding rods are not solid copper, but rather copper plated steel rods.