How can I obtain dragonstone armour or stannis armour? It seems it is not available in any shop.
It drops but it's rare.
I got some Stannis Plate (or something like that) when I took Dragonstone. There were 150 Queen's Men in there and they wear the SP I believe.
That's the bad thing about this mod. You have to either choose to get prisoners or choose to get loot.