[serious] where to buy plastic covers for books?

does anyone know where to get those readymade plastic covers for books? tried a few popular bookstores but they said they only sell it once a year (at the end of the year)...

or does anyone know where to get hard sticky clear plastic? (instead of the soft clear plastic type which needs to be taped together...) my sec sch library used to wrap books with that... the plastic can just stick directly on the surface of the books...