Many of the bigger hardware stores have drawer boxes loaded with all kinds of bolts, nuts, cotter pins, woodruff keys, and almost any other kind of hardware that is frequently in need of replacement. They even carry motor brushes for small electric motors. If you take your router there and explain what you need, they can likely come up with something that will work and it may even look exactly like what is shown in the manual. The big box stores have similar assortments in drawers, but they only stock what sells frequently. The better stocked local hardware store are more likely to have what you are looking for.