Try the young Adult section of your library (aka Juvenile).

Many libraries have to 'rotate' their books to keep what is on the shelves fresh. Sometimes the library just moves the books to the basement stacks - other times they get sold at 'book sales'.
I have noticed that the Andre Norton juveniles, like the Mary Norton juveniles, stay on the shelves much longer.

PS: I only saw but one Andre Norton book on the shelves of the library when I was a child. My library had very few Science Fiction books in those days. Today your library should have a good Science Fiction section, and if they don't have Norton on the shelves, check the catalogue. Her books are probably so desired that they are probably all OUT! If not, suggest to your library that you'd like to see some Nortons. Libraries actually do listen to their patrons.