Pretty much any trouser or knee-high sock will do. But why not go with one that has a little color!
I wouldn't recommend a wool sock, as it might feel scratchy. And make sure the sock hasn't lost so much of its elasticity that it won't stay on your upper arm while running.
Bring up the bottom of the armband over the phone, so that it holds the phone snuggly. And that's it! You have the best darn DIY armband for running out there!
Now get running!