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
