I'm afraid you'll have a real problem trying to find different
shapes of copper wire. Round wire is everywhere, especially at the
home improvement stores. Square and half-round, however, is
completely another story. Brass wire you may be able to find at
www.wire-sculpture.com, Preston Reuther's site. He will probably have
whatever variations of copper that are available too.
Sterling silver wire is not that terribly expensive. I pay about $13
an ounce, and depending on the gauge, that can be a lot of wire
(approx. 20ft in 24 ga.). Copper tarnishes so badly, so quickly, that
I just won't use it unless a customer begs for it. All of the major
supply houses carry sterling silver and is very easy to find.
Good luck in your search!