Title says it all, where can you get them?
I remember fighting a few draugirs on Iorveth's path when Phillipa led me through the fog, but I didn't see any on my trip this time around (I couldn't pick them up anyway since enemies spawn endlessly in that area).

I'd like to be able to craft the oathbreaker's armor before close to the end of chapter 2.

But it's correct, the Bullvore in the mines has the same drops as draugirs, and you will most likely get the armour piece from it.

You should be able to kill at least one draugir and get the armor fragment, right when you start Chapter 2 and defend Henselt from the ghosts in the mist. That's basically the earliest point where you can get the item (after that you have to wait until you can get through the mist again or reach Vergen). All you need after that is a few harpy feathers (easily obtainable) and tons of cash that you can win via poker/arm wrestling.

You also need it for the cursed armor on the dark difficulty, which is way better.