Almost every library I've patronized in the last four decades has used the same basic set of icons to delineate the various genre ghettos: a skull for mysteries, a stylized atom or a rocket ship surrounded by "atomic" rings for SF, and so forth. Most of them even seem to use the same company, with red ink on yellow stickers.
Why can't I find these icons on the web? I find several places selling library genre stickers, but not the classic old red-on-yellow designs.
I know they're OUT there -- I see them at my local library all the time, on brand new books.