稀少性を表現

hard to come by/rare

one and only (idiom); rarely seen/unique of its kind

unprecedented and never to be duplicated/the first and the last/unmatched/unique

just a very few (idiom); tiny number/not many at all/You count them on your fingers.

rare as morning stars (idiom)/few and far between/sparse

better to have nothing (than substandard choice) (idiom); would prefer to go without than accept shoddy option

unprecedented

lit. the iron tree blooms (idiom)/a highly improbable or extremely rare occurrence

one thread, one hair (idiom); a tiny bit/an iota

one in a hundred/cream of the crop