crème de violette
Apparently popular in the 1800's, creme de violette was used in a variety of cocktails for both its floral flavor as well as the interesting hue it would bring to a drink. Its availability slid drastically in the 1900's to the point it was not only impossible to find in America, but often very difficult to find even in France, where there used to be several competing brands.
With the resurgance of the classic cocktail, several companies are now working on better distrubution of existing products as well as bringing new ones to market as well. It might still be hard to find in general purpose liquor stores, but you should be able to find it in specialty liquor stores, or online, without too much trouble.
The following recipes on this site use crème de violette:
crème de violette,