The primary place to get mods/upgrades for weapons/armour is from the commendation vendors on each planet, or from the GTN by buying them from other players who have crafted these items.
Another way is some quests actually reward you with a choice of upgrades. Usually armouring being the main one I've seen, with enhancement being next, and on the rare occasion barrels (I suspect for saber users you'd see hilts as rewards).
It is VERY rare to see a mod drop from a mob you've killed. The less than half a dozen times I've see it happen, its been a purple mod, and only once has it been a mod I could use, and even then, I had access to something that was the same, if not better in a blue already slotted.
I think you'd have a far better chance of seeing a fully slotted item drop, and be able to canibalize it by removing mods from it. I've seen two "orange" sabers drop in my times on Drommand Kaas, and pulled a couple mods from those to use on my sith warrior.