FTW, libraries offering per-diem work is fairly rare. I'm in a metro area with 8 public library systems and 7 universities, but only 1 of the public systems and one community college offer that kind of shift work, and not all that often; they basically use it only to cover when regular staff are on vacation or out sick.
i would be surprised if any libraries around us offer it as well-the public have a LONG waiting list to become a volunteer and the college's are staffed w/ft permanent staff and work study students.