Update: With the release of patch 1.22, the developers have added a crafting diagram for Dark Iron Plate to the game.
Witcher 3 Dark Iron PlateThis method is somewhat expensive, and it requires the Hearts of Stone expansion. If you need more cash, you could try out the Infinite Money Glitch. Once you’ve filled your coffers, travel to Upper Mill in the north-eastern corner of Velen. Talk to the Ofieri merchant there and buy the Ofieri Tunic for 835 crowns.
Go to a craftsman (the one behind the windmill, for example) and dismantle the tunic. You’ll have to pay 100 crowns and you’ll get one piece of Dark Iron Plate. The merchant will restock after four days – you can just meditate and buy another tunic as soon as it’s available. As long as you’ve got the money, you can repeat the process indefinitely.
Props to TheAnimatedDog for doing the detective work.