just look for it on nexus, i searched "no essential" and found 3 results. it looks like these mods just edit each npc in vanilla to not have the essential flag set, so it will conflict with other mods. Im not sure right off the bat if wrye bash can fix this, so I dont think i would advise using them. Also killing some essentials will break quest and halt your progress.
If you dont care about the incompatibility or quest issues those mods do exactly what you want. The only problem is it wont change any npcs from other mods to non-essential.
edit:
just looked up essential on nexus, http://tesnexus.com/downloads/file.php?id=20509 mod adds a spell you can use on each npc to change their essential status.