General synonyms:
* Uncommon
* Scarce
* Infrequent
* Unusual
* Seldom seen
* Hard to find
More specific synonyms:
* Precious (if referring to value)
* Exquisite (if referring to quality)
* Exceptional (if referring to uniqueness)
* Elusive (if referring to difficulty in finding)
* Endangered (if referring to a species)
* Antique (if referring to an object)
Antonyms:
* Common
* Abundant
* Frequent
* Typical
The best synonym to use will depend on the specific meaning of "rare" in your sentence.