scarce as hen's teeth
Also, scarcer than hen's teeth. Exceptionally rare, as in On a rainy night, taxis are as scarce as hen's teeth. Since hens have no teeth, this term in effect says that something is so scarce as to be nonexistent. [Mid-1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.