You are thinking of the wrong Pokemon. Magneton doesn't evolve that way- Scyther does. You can obtain Coronet rocks as mission rewards and you can buy them @ Kecleon Shops.
Here is a WM (Wonder Mail) code for a mission that rewards you with a coronet rock. And honestly, I think this would be the best answer if I were you. This works for EOD as well.
T%1N TK-7 2CY2 J8J5 NR#H 4Q&R