Here is a previous conversation
on "woodruff" keys that I think has the basic answer to your question.
Also, this Common Item Page
] provides the dimensions for the various common "woodruff" keys used for your application.
If you use the info from the above thread:
If a person who needs that woodruff key does not have a woodruff key, Denny, what part-number or dimensions would you send them (send me) with when they go to their FLAPS/etc?
Width-3/16(.188) In., Length-47/64(.728) In., Height-19/64(.301) In
Length in Inches--3/4
Width in Inches--3/16
It appears the one you may be looking for is GM# 106751 (see screenshot below).
Listed as available from all kinds of vendors online. Best price I found in a short search was from the Filling Station
Hope this helps.