Expression for a rare find?

Solution 1:

I guess you mean 'difficult to locate/find', rather than 'rare'. The commonest idiom for objects difficult to locate is needle in a haystack.

Red cars, for the man, were like needles in a haystack.

This means the man found red cars very difficult to spot.

There are several related idioms for things that are hard to chance upon. A couple of them (also mentioned here):

• scarce as hen's teeth
• black cat in a coal cellar

Solution 2:

Seeking El Dorado can mean looking for something that may be just a legend or an illusion or for something that may never be found. According to the Wikepedia article on El Dorado, the legendary "Lost City of Gold":

El Dorado is also sometimes used as a metaphor to represent an ultimate prize or "Holy Grail" that one might spend one's life seeking. It could represent true love, heaven, happiness, or success. It is used sometimes as a figure of speech to represent something much sought after that may not even exist, or, at least, may not ever be found. Such use is evident in Poe's poem "El Dorado". In this context, El Dorado bears similarity to other myths such as the Fountain of Youth and Shangri-la.

This dictionary entry under the word rarity has a list of comparative quotes that might have something you could use, such as:

Rare as a man without self-pity —Stephen Vincent Benét

Rare as a politician who lives up to his campaign promises.

[Money … was as] scarce as frogs’ teeth, crabs’ tails or eunuchs’ whiskers —Pat Barr

(Barr’s colorful multiple simile refers to the scarcity of money in Korea during the late nineteenth century when the heroine of her book, Curious Life For a Lady, was there.)