Apparently these are next to impossible to find in stores, and the local lawnmower repair shop wanted a lot more to sell me just one by itself.  It was worth it, especially since I'm handy, to buy the three-pack from Amazon and replace my broken flywheel key myself.  This solved all my issues since hitting a rock warped my blade and sheared my flywheel key in half.  So glad these were available, and I was able to fix the mower myself for less than $30!!