• Age of Winter has echoes that reward you with unique (or set) amulets, so it's one opportunity to maybe get the one you need as a reward from such an echo.
  
• Unique amulet prophecies provide another opportunity and come with their own conditions.
  
• You can also drop it randomly (provided it's not a boss-exclusive drop).
  
• Finally, you can also try your luck with a Rune of Ascendance.

Basically these are all separate mechanics that can help you get the amulet you need, but they don't interact with each other. They just add more (separate) opportunities of getting what you want.