You can get ectoplasm in The Elder Scrolls IV: Oblivion primarily by defeating ghosts and other spectral enemies like wraiths, as they drop ectoplasm as an alchemy ingredient

. Here are some reliable ways and locations to obtain ectoplasm:

• Quests with ghosts:
  • The Forlorn Watchman quest in Bravil and The Ghost Ship of Anvil quest both feature ghosts that drop ectoplasm

* The Fighters Guild quest "Drunk and Disorderly" in Anvil docks involves killing ghosts on the ship "Serpent's Wake," which gives you 5 ectoplasm needed for the quest

• Dungeon farming:
  • The Santor dungeon above Coral has ghosts and wraiths that respawn and drop ectoplasm, making it a good farming spot

* The chapel under the castle in Anvil also contains ghosts that drop ectoplasm

• Buying ectoplasm:
  • Some alchemy shops in Skingrad, such as "All Things Alchemical," and other alchemy vendors in West Weald and the Imperial City market district sometimes sell ectoplasm, but it is rare and not guaranteed in their inventory

• Static item:
  • In Oblivion Remastered, there is a static ectoplasm item on a table in "All Things Alchemical" shop, which you can take, but it is a one-time source

In summary, the easiest and most consistent way to get ectoplasm is by killing ghosts in specific quest areas or dungeons like the Santor or the Anvil chapel, with the Fighters Guild quest in Anvil being a convenient source of multiple ectoplasm pieces

. You can supplement this by checking alchemy shops for rare sales of ectoplasm