You are in luck. I was able to find a pdf version of Susan Sontag's essay for you in the online journal Daedalus. I've included the link to it in the sources section. It is not a short story but an essay, and the full title is "An Argument About Beauty."

The essay is also printed in Sontag's last book, At the Same Time: Essays and Speeches, which she wrote before her death from blood cancer in 2004. You can preview that book at books.google.com or at amazon.com.

I hope this helps you.