The commonest idiom for rare objects 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](http://english.stackexchange.com/questions/15620/more-idioms-like-needle-in-a-haystack-relevant-to-hidden-hard-to-find-items)):

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