In the Dwemer underground city ruins. If you go everywhere in the labeled dungeon there (forgot names right now) there is a Dwarven Bow with Soultrap on it.
It's kinda like an optional quest item, you can turn into an old man whose owns the Dwemer museum there. But instead of turning it into him for cash, use the "enchanting" table right near him. Trust me it works, that's how I have Soultrap now on most of my weapons.
