We have a metal recycling company that buys scrap but also sells it. I stop by there once or twice a month looking for solid copper rounds and they have a 4' by 4' box of assorted copper scraps which include the ground wire mentioned. It has a natural hardness to it as some of it has been in the ground for many years.
I have also found copper rounds as large around as 3". Used copper costs me $4.80 a pound.