Some people use aluminum gutter nails with apparent success. They aren't all that expensive and are easier to come by than copper rod. Copper ground wire works well but, as suggested, should be twisted, which both hardens and straightens it. You won't know whether you like aluminum over copper until you've used both, so if aluminum is readily available grab some and give it a try. There are those who swear by solid aluminum boppers, as well, but I don't care for them due to the reduced weight. They are cheaper than solid copper, though.