Few and far between.
Few and far between.
'Few and far between' means occurrences are rare and occur over wide intervals in time or distance. It conjures up the image of searching wide spans for sparse treasures, highlighting the rarity and value of such finds, whether they're opportunities, events, or people.
Opportunities like this are few and far between.
Such opportunities are very rare.
Rainy days are few and far between in the desert.
Rainfall is extremely uncommon in the desert.
Good books on this subject are few and far between.
Books on this specific subject are very hard to find.