@dextersarus eeveeteam’s guide here is one of the best ways to go about searching for these difficult unlimited cards. And the earlier you get comfortable with possible daily searches, the better (luck favors the prepared!) 
If I remember right, I found the circlehunter’s unlimited Haunter searching just “ゴースト” (very general search term of just “Haunter”) on Mercari Japan and trudging through the listings. I ended up finding it in a small Haunter lot that had no mention of XY at all.
I also like how this collection thread has become the de facto hang out for all the Haunter species collectors 