If you can't find old pennies, hardware stores have lots of copper items. Even a small slice from copper tubing has plenty of copper in it to protect a bird-bath.
If they don't sell copper tubing by the foot, there should be some cheap copper fittings or small spools of copper wire in the electrical department. Maybe some plumbing fittings are copper and not brass.
You might want to avoid brass since it always contains zinc, and sometimes 1-2% lead or some aluminum. Whether or not those are toxic, they will slow down the copper from oxidizing and preventing algae.
