How do I get weapon mods?

I know that there is a random chance to get some loot from enemies when you kill them. However, many of my soldiers don't have modded weapons due to not having found enough.

Other than looting, is there any other way to get weapon mods? Can you build them?

It seems logical to me that XCOM should be able to build a scope for a gun but I've not seen anywhere to do so.

Solution 1:

Weapon mods can not be built. But:

• The black market will often offer you weapon mods in exchange for intel.
• The "Vulture" perk in the Guerilla Tactics School will increase the number of mods you find.

Another thing you can do which doesn't give you more weapon mods per se but increases their availability is getting the continent bonus which allows you to remove mods from weapons and put them on different ones.

Also keep in mind that when a soldier with a modded weapon is injured, you can temporarily equip them with an unmodded weapon to make the modded weapon available for another soldier. So you don't need enough mods to mod the gun of every soldier, only enough for a full squad.