I've seen them in Garden of Eden and in the grocery store at Chelsea Market.
Garden of Eden
7 E 14th St, New York, NY 10003
You Americans should try out finger limes
They are are very amazing native Australian fruit that are fairly unique
Oblong in shape - you squeeze out these caviar-like little balls of tarty lime/grapefruit goodness.
Extremely good on Icecream - really really good counter agent to the sweetness of . Apparently works well with Champagne
Check it out - I'm suprised the gastronomy types dont use it more