Is there a best place to search for the Daedra banishing enchantment?
Solution 1:
The lowest tier of weapon that can have banish is Elven, they start to show up at level 22. Check the stores and the boss chests in dungeons. Doesn't matter where you look.
Solution 2:
In both of my playthroughs, I have found it on an item from a merchant first. Check all of your merchants, particularly the smiths, as they have a large array of weapons.