you can get it in two places i think. In the college i cant remember his name but he trains conjuration and talks about necromancy. Secondly I think (not 100%) the mage in Windrun's castle sells it too.
Conversly if you put enough points into conjuration you can get a perk that all bound weapons automaticlly steal souls. This is what I do because first you get an awesome ghost sword that better than most steel ones, and you lvl conjuration and one hand
simultaniously and stealing souls its a triple win.
Here is what I did to trap souls and it's worked great for me.
In Whiterun I found a bow for sale in the Drunken Huntsman that had soul trap on it. I have also seen the Blacksmith sell these items, but the bow in the Huntsman seems pretty common. I bought one of those bows and took it to the arcane enchanter and destroyed
it to learn the Soul Trap Enchantment. I then took a Skyforged Steel Sword (any Melee weapon will work) to the Enchanter and enchanted it with Soul Trap using a Common Soul Gem. When you are doing the enchantment, and you pick the Soul Trap ability, put the
slider all the way to the left. This will reduce the soul trap time to 1 second and give you the max possible charges. One second is fine because it traps the soul as soon as you deliver the 'death blow.' Craft your blade and you're off. You can then put some
empty soul gems in your inventory and go out in the area around Whiterun and kill Wolves, Mud Crabs, Wildlife, etc with your Soul Trap blade and every kill will fill a gem. Just remember, regular soul gems can only hold the soul of creatures and not humans.
You need Black Soul Gems to trap human/humanoid souls. This method should keep you with a steady supply of regular Soul Gems though. Hope this helps you.