John McIntosh has them listed on his site; I don’t know if he actually has any. 47-21164 is what it shows in my parts listing. Fits the KG-7, KF-7, and KG-4 .
http://www.johnsmcintoshvintagemercuryoutboard.com/Impellers.html
I have read that an impeller for a KG-7, KF-7, KG-4, and some Wizards (Mercury 47-21164) can be made from a Sierra 18-3051 (OMC 395289). You need to modify the height but that should be easy enough to do. Verify this information; it is in my notes.
Odd that OldMercs does not respond.