Here's a list of who drops what:
Purebloods - Personal Database
Ninja Gals and some Raiders - Rare Earth Elements
Academy - Integrated Devices
Guild - Durathread Mesh
Church - Force Circuitry
Sectoids - Engineered Biomatter
Power Armor enemies - Power Armor Parts
Mercenaries and Megapol - Synthmuscle Mesh
Ethereals - Star God Robe
You usually get at least one of those per dead enemy (unless the corpse is lost either via overkill or something exploding the corpse away), and of course you get that plus more when capturing them and then dealing with them (robbery, slavery, interrogation).