I'm probably a bit too late to contribute my advice XD
But the good thing about Magnet cosplays is that you can be creative. Black heels and striped stockings are what I would go with. You could also do ankle boots, or even knee-high boots with them. Basically any black shoe. Even convers might work. My favorite socks to go with this outfit are black and white stripped stockings (I got mine from a lingerie shop, but you can probably find them at Hot Topic or Spencers). If you want to be really creative, I wore a Magnet-inspired outfit once using the striped stockings with fishnets over them. It really depends on what look you're going for:
colorful: striped stockings with different colored fishnets (one black, one pink or blue, matching the color of the headphones. look around; you'd be surprized where you can find them)
fun: striped stockings with black fishnets
cute: striped stockings
daring: fishnets
pretty: simple nylon
these can work with really any black shoe: flats, heels, pumps, boots, convers, etc
