Hey Tell, I know why your looking for the large copper billets,,,,,,,,You've been watching Emory bust up the big ones.
Sorry we missed you guys out there, Had to tend to a friend in crisis hear on the home front.
As far as the copper billets I have them keeping an eye out for me down at the scrap yard. You might also check your local steel scrap yard. Often they get some in. And much better then buying new.
Another trick that I found is to use a steel shaft for most of the weight and cap the end with a large copper end cap. I get one a little bigger then the shaft then melt in the lead around the shaft.