It is surely a good suggestion to use old electrical wire for
alloying, but none the less it is not the only source for getting
good quality Copper.
There are a lot of vendors who supply "electrolytical grade" copper
which is at least 99.99% pure ( and it gets its name as it is
electrolytically refined copper). Also there are other finer grades
available, the best being ( which I know of ) OFHC grade copper i-e
Oxygen Free High Conductivity Copper. As it contains least possible
impurities (as also Oxygen ) there is very little scope for
contamination of the alloyed gold. To get the best possible result
of this alloying metal it is better to melt the alloyed gold in a
A few vendors who come to my mind are Belmont Metals Inc., based in
Brooklyn, N.Y., and all the other jewellery manufacturing consumable
suppliers should be able to take care of your requirement, though
they could charge you a premium for small quantities ( you can only
buy so much to alloy with gold ).