Try Bailey's or any other logging/forestry supplier. They usually have chipper knives for far less than you will pay for replacements from the manufacturer.
If you have the bobcat chipper, it is made by bearcat. Knowing what I know now, i would have saved 3 or 4k and bought the Bearcat. I hate that I have to shut the chipper off and start the Toolcat to move the chipper 5 feet! That's ridiculous but I haven't found a way to bypass the control box.
