Learn English – Expression to say that you did something better than before

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

This phenomenon is called "The Halo effect".

The Halo effect is an effect where the reputation or the trust of someone or something builds up so much that everything done or said by that person/thing starts appearing to be divine, and the best.

The phrase of 'Halo effect' can be used in relation to a thing as well as a person.

The opposite of 'The Halo effect' is 'The Horn effect', wherein, anything done or said by a person/thing is considered to be untrue and unbelievable.

So, the phrase that you are looking for is 'The Halo effect'.