Reddit seems to believe that you can get them from the sack of Mr Sacks. That is definitely not the only source, because I’ve never done that and I have 3.
The other Wiki says they can be obtained from Written in the Glim. My unreliable memory says they were seasonal tie-in content for something related to Sunless Sea or Zubmariner, but I can’t find any confirmation of that.
If Ambiguous Eoliths are Fate-locked, then the text you’re trying to obtain is also Fate-locked so it shouldn’t be on the Wiki. If I were in your position I’d be trying to get it just for my own sake, but I felt I should mention that.