KitchenAid have not made these waste disposals in over 20 years. Most parts ran out years ago. We are not be able to get any more.
If you need a part including the magnetic stopper (magnstart) one you have several choices:
a) Don't use the waste disposal any more
b) Try to fix your existing part
c) Make a replacment (we'd like to hear if you do)
d) Find a second hand one on ebay (they don't come up often)
e) Replace the waste disposal with an alternative make.
We have been asked many times to recommend an alternative. After much research we have concluded that for the UK at the time of writing that the Insinkerator Evolution 200 waste disposal is the best replacement.