Chris - The original woodruff key for your application is GM# 106751 (Key, woodruff - 3/16" thick x 9/32" tall x 3/4" long #9). If you search the web for GM# 106751 you will find lots of sellers/sources offering them, either NOS or new replacements with prices ranging from as little as $1.15 (not counting shipping) to upwards of $9 a piece (also not counting shipping - OUCH).
BTW - It's the same key used for the crank and cam gears if I'm reading things correctly in the "Parts" resource books I have.
