You work out what it is you have - which one assumes to be a document shredder; you don't say but that is the category in which you have posted
You also need the make and model of shredder which, again, you didn't let us know.
Once you have all the information, why not do a search for "Make spares" (replacing 'make' with your own info) and start rooting around for spares for your particular model.
You may even get some mileage from the manufacturer, assuming you have a machine from a reputable manufacturer (as opposed to pile 'em high and sell 'em cheap Chinese tat).
If you still want other people to do your searching for you, please re-post, giving enough information for someone to do so