The following libraries mentioned the Lorax's challenge:
    Kansas City, Kansas Public Library:
During the banned books week the Lorax was mentioned as a challenged book at this library.
    Longview, Washington Public Library:
The Lorax was discussed in an article by the Longview, Washington Public Library about challenged books.
    Mansfield University Library:
Mansfield Unversity has a collection of banned books in its library that includes the Lorax.
The following websites mentioned the Lorax's challenge:    points
This source compiled many banned books on one list and the Lorax is on this list.
This site has a table that is composed of banned books. One of the banned books mentioned is the Lorax.