Once you actually get to heavy armor surcoats for heavy armor stop being sold, buy a few ones while you still have them in the shop.
I finally reached heavy armor and I can't any surcoats anymore.
Okay, once you reach level 20ish the heavy surcoats stop being sold, the robe ones though are still sold.
If you can't find them being sold you're going to have to rely on chest loot, there are plenty of chests in the game.
They stop spawning both in shops and in chests once you get past a certain level.
I had to lower my level to 10 via cheats to get them to spawn in shops, buy one, raise my level again.
I got one as a quest reward around level 25, there's a quest from the New Ashos billboard where you have to kill groups of beavers.
So far I could only see one surcoat that let me wear it with my heavy armors.
I found it on the noob island where my orcs rescue team lives. I ran around after I got to level 15 and saw harder mobs there. They drop good stuff and even got a few side quests while exploring the remaining of the island. One of the chest they had on that island contained the item level 10 surcoat that I can wear with my knight armor set and it can be upgraded.
The rest of the surcoats I encountered in the game so far are only designed for robes.
Yeah, this is certainly something that MUST be fixed in a future patch. It is similar with me trying to find certain skills books that don't seem to be anywhere.
I found every single skill book in the game besides Fire Strike and Battle Cry (both melee) from dungeons and chests with static items from the prologue and first chapter. I did see Fire Strike being sold by a vendor in the 2nd chapter.