Sad Essences can be obtained from interactions with Sims. Being Mean is one main way to obtain these essences. However, most Sims drop Angry essences instead of Sad when being mean. Most Spooky Sims should drop these essences when being mean to them. If you do not want to be mean to any Sims, try being nice to Vic Vector, Elmira, or Goth Boy. All three drop Sad essences when Being Nice no matter your friendship level. It can also be obtained by Being Nice to Sims who you don't have a good or average relationship with for some Sims.