More idioms like "needle in a haystack" relevant to hidden/hard to find items? [closed]
Are there more idioms, sayings or phrases similar to "needle in a haystack" that are relevant to hidden objects, or difficult to find items?
Also interested in similar nouns relevant to the somewhat oppositional concepts like "needle" and "haystack" or "3-leaf clover" vs. "4-leaf clover" where one object is significantly harder to find.
I have a one-word answer:
Waldo
"Looking for a black cat in a coal cellar"?
If you take "hard to find" as being "very rare/non-existent" …
"Scarce as hen's teeth."