I think the main reason you're having trouble finding the print is because Andy Warhol never made one called "The Lipbalms" -- it's a gag prop made up specifically for this play.

Like Warhol's portraits of Marilyn Monroe and Jackie Kennedy, he supposedly did a similar study of a couple named Sidney and Sylvia Lipbalm -- a prominent proctologist and his wife -- which hangs on the wall of their apartment.

You can use a portrait of any two people you like and have your prop department rig up the fake. Good luck.