Polycarbonate is a resource in the The Division 2 that is not only used in the crating of weapons and gear, but also to upgrade the Crafting Bench where you craft these items to higher World Tiers.
You also need quite a lot of it to complete various different projects that you find in the game world.
It’s a fairly rare resource, and the most common way to get it is to kill enemies in the world who will occasionally drop it, alongside other resources and gear. But this farming method yields very little Polycarbonate, and takes up a lot of time.
The second way to get it is to take over the numerous different Enemy Control Points scattered around the map. Once you do this, each of them then allow you to access their Supply Rooms, which contain various different materials in bulk. The higher level a Control Point is, the more resources you get from it.
Another way to get Polycarbonate is to fast travel to the different Safe Houses on the map and just wander around their immediate vicinity to locate slightly glowing containers that have different resources in them.
You can find these containers scattered all around the map to be fair, but I found them more frequently around Safe Houses.