ACE Hardware sells 1/2" Aluminum pipe, 6 feet long. Spring steel wire from Hobby Lobby with a little bit of cotton ball wrapped loosely with very thin thread. (old school but, works well and you can make a dozen darts in two minutes).
Spring Steel wire is also my favourite. Specifically piano wire. You can buy it by the roll or in pre-cut lengths approximately 30-36 inches long. Can't remember the exact length as it has been so long since I last bought any but I can remember how indesctructable this stuff is. It won't even bend under normal use......it just springs back straight.
I have a bunch of Flag Wire that I got while down in Mexico that I ordered off Amazon that GKU recommended to me so I will give it a try this winter. But high carbon spring steel wire is my all time favourite albeit a little expensive, I've never had to throw a shaft out because of bending or damage so it's cheaper in the long run.