Good and bad news. The good is, that basically all 84-89 have the same knob. Bad news is, it is sold with the shaft, and you have to gain access to the back of the switch to trigger the release to get the old shaft out. Some people use a small ice pick type tool and trigger the snap tap inside the knob and yank the knob off the shaft. However, this will damage the integrity of the retainer and cause the knob to fall off if messed with again. That may be what happened to yours.
I just happened to receive a new Corvette America catalog in the mail today. (800-458-3475). They list the knob and rod as part #34225 and ask $39. I don't know if it's a good deal or not , I just happened to be holding their catalog when I read your post.