I believe the RC12YC should be available and any parts store. At least I've not had a issue. It's basically a magnum plug (just smaller hex socket than the original RN12YC)
The RC12YC is definitely available like you said. I wanted to get the electrode further into the chamber, without having to do any blending on the plug hole (I forgot about it until I had the heads together).
Champion RC12MC4 and autolite 5224 have a .875 reach vs the .750 of the RC12YC. But the champions aren't available in the parts stores local to me.
