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Check out Gene at Gene's Kar Shop, who is a Forum Vendor and he may be able to hook you up with a used one. Alternatively, contact Fred Beans for a new one or you may be able to Post in the For Sale section for one.
(Look to the left, scroll down and you'll see their logo.)
From the diagram I have in front of me it looks like the part number might be:
10404727
Which is described as
Housing (ignition and start switch).
I can't tell however if that includes the ring that you are looking for which may be part of the ignition switch itself. I agree with contacting Gene at Fred Beans (new parts), or Gene's Karshop (used parts) to see what they have to offer.