Question: where are soul trap weapons in skyrim?
Answer: Weapons with soul trap enchantment are often not labled soul trap. ... 
The Mace of Molag Bal (unique item) is a pretty decent soul trap weapon, which you get as reward for a quest given by Molag Bal in the abandoned house in Markarth. ... 
Wait in 2 day cycles in whiterun or dawnstar shops that sell magical weapons.