What is the source of the allusion in the sentence below?
Isabelle entered the bustling, aromatic spice market in Istanbul and said to her travel companion, "We're not in Kansas anymore."

 - a movie
 - a fable
The source of the allusion We're not in Kansas anymore is a movie.
In the 1939 film The Wizard of Oz, Dorothy, a young farm girl from Kansas, finds herself in Oz, an unusual place that looks nothing like her home. She says to her dog, "Toto, I've a feeling we're not in Kansas anymore."
The allusion We're not in Kansas anymore means we're in an unfamiliar place.