I just defeated plantera and I wanna get a horsemans blade But I feel like I'll need more than one try to get it. What's a fast way to get Ectoplasm?
Ectoplasm is dropped by Dungeon Spirits, which have a chance of spawning whenever you kill an enemy with more than 100 health in the Dungeon after killing Plantera. Just stick around the dungeon and kill anything that moves and you'll naturally find Ectoplasm, as well as other strong weapons and accessories like the Paladin's Hammer.
That said, hardmode Dungeon is a very dangerous place, so be prepared with your best equipment. Maybe kill Plantera a couple more times until you get useful weapons, and make sure you have the best armor available to you, which is probably Chlorophyte, Shroomite, or Turtle depending on your choice of weapon.
I play in expert mode and I figured out that you can kill the blue slimes from the slime king and it gives tons of ectoplasm. So if you're in expert mode, try farming king slime